A pet form of John, Johnny is used, with modifying word, to designate a person, especially a man, of the type, group, profession, etc., specified. For example:
– Johnny Arab is an offensive generic name for an Arab man;
– Johnny Crapaud is used to personify France or the French people, or to designate a typical Frenchman;
– Johnny Foreigner is used to designate a foreigner or to personify foreign people.
—Cf. also Joe Six-Pack in American English, and Joe Bloggs and Joe Soap in British English.

Likewise, in British English, Onion Johnny, also Johnny Onions, was a generic name for an onion-seller from Brittany, especially from the town of Roscoff, who sold onions door-to-door around the coasts of England, Wales and Scotland. Such onion-sellers were most common in Great Britain from the early to mid-20^th century.

However, in Onion Johnny and Johnny Onions, Johnny may have specifically originated in the French forename Jean-Yves, at least according to the unsigned review of Goodbye, Johnny Onions (Redruth, Cornwall: Dyllansow Truran, 1987), by Gwyn Griffiths—review published in The Courier and Advertiser (Dundee, Angus, Scotland) of Saturday 6^th August 1988:

Many [Breton onion-sellers] were called Jean-Yves, which quickly turned into the nickname “Johnny” on British doorsteps.

The earliest occurrence of Onion Johnny that I have found is from Echoes of the Week. Contributed by Peter Playfair, published in the South Wales Daily News (Cardiff, Glamorgan, Wales) of Monday 12^th November 1877:

The Breton men and boys with their monotonous cry of “Inyauns,” are now looked for as regularly as autumn comes round, though it must be confessed that they are not quite so civil as they might be, nor as they used to be. Not content with standing in the streets near the market, on a Saturday, they now perambulate the town and walk into private houses, wherever they can find an open door, with a want of ceremony that says little for their French manners. The way in which they handle door knockers is also far from pleasant to the nervous housekeeper. Some time ago the borough authorities prosecuted some basket girls for selling vegetables in Roath, without having previously paid market toll. I should much like to know whether the “Onion Johnnies” pay toll before they start hawking the town? The persistent and annoying manner in which “Johnny” puts his foot between the door and the jamb, when a female answers his tremendous knock, and roars out “verra sheep,” is quite startling to the nerves; but I have been led to say a few words about “Johnny” and his doings from a scene I witnessed on Saturday. The small boys of the town seem to consider “Johnny” as lawful prey for teazing, and about half-a-dozen youngsters were thus engaged on Saturday evening, when “Johnny” suddenly dropped his onions, ran after the biggest of his tormentors, and administered a kick with his heavy wooden sabot in the posterior of the unfortunate lad, which sent him howling and limping away. In fact, “Johnny” is far from being harmless, and it is well it should be known that his temper is not to be trusted. I remember that last year one of them was sent to prison for kicking a woman violently on the leg. No one can justify the Cardiff boys being insolent to the onion-men, but they, in turn, should be taught to use good language. The humbler class of foreigners seem to pick up “unmentionable” English words, and they contrive to use them whenever they can’t do as they wish—especially to females. Cannot Chief-constable Hemmingway read one or two a lesson?

The earliest occurrence of Johnny Onions that I have found is from The Brecon and Radnor County Times (Brecon, Brecknockshire, Wales) of Friday 2^nd November 1894:

Observations.—The voice of the onion man is heard in the land: the clang of wooden shoon [= shoes] reverberates through our classic streets. A wily merchant is the little Breton lad, and persistent—oh! very persistent. He calls out at the kitchen door, “onions, missis, ver cheep.” The lady of the house answers “no,” but that won’t do for him, and in piteous accents he begs her to come and see them, lift them—smell them, saying he will be beaten if they are not sold. This last dodge succeeds at first, but a frequent repetition makes householders wary. “Johnny Onions,” as the children call him, is a bit of a nuisance. But that is no reason why he should be bullied by rag-tag lads. If he is a stranger within our gates, that is no justification for pelting him with garbage.

This very interesting article was published in the Milngavie and Bearsden Herald and Dumbarton County Courier (Milngavie, Dunbartonshire, Scotland) of Friday 30^th March 1928:

ONION-JOHNNY.

At many doors all over seaside towns in England, Wales, and Scotland, and far inland, is appearing just now that smiling-faced young foreigner with his vegetable merchandise known as “Ingan ¹ (Anglice, Onion) Johnny.” His engaging frankness and good temper, his persistency, and business flair more than compensate for his deficiencies in the English tongue. Of this last the totality on his first visit to our shores as the youngest member of the onion-sellers’ equipe (gang) is the eloquent phrase, uttered with beaming smile and a show of perfect teeth, “Onyons, sheep!”
He is a familiar figure in Britain and the Isle of Man, and sells hundreds of tons of onions at a price which, considering the value of the franc, safely places the growing of onions in Britanny for the markets on this side of the Channel as one of the most lucrative branches of French agriculture.
The two great centres of the onion-growing districts in Britanny are Morlaix and Roscoff. The latter place may safely be called the onion metropolis. This flourishing little town is situated on a promontory of the Breton Department of Finistère, about 100 miles southeast from Land’s End, in Cornwall. Roscoff—which, by the way, has the same derivation as Rosyth ² (Haven Behind the Promontory)—is a typical Breton ville, where the old Breton tongue is universal except among the French-speaking official class. It has a special interest for Scottish people, for it was from this old seaport of France that the young Queen Mary of Scots embarked for home after her childhood spent in La Belle France. All around in the neighbouring “pays,” blessed with an extremely mild and even warm climate all the year round, where centuries of laborious industry and intensive cultivation have produced of the once shallow and rock-cumbered soil, rich tracts of land worth its weight in gold, the Roscoff “cultivateurs” annually raise thousands of tons of the homely health-giving vegetable.
Here for many generations has existed an almost Communistic system of co-operation in industry. All middlemen are eliminated by the adoption of family co-operation in every operation—from the sowing of the onion seeds to the book-keeping and division of the proceeds of the sale of the vegetables. A “famille” may consist of 150 or more members, all generally relations—brothers, cousins, second cousins, and to the fifth and sixth degree german. It farms and owns the land—from 15 to 50 acres—under intensive cultivation. All the tools, drying-racks, and sheds necessary for the cultivation and storage of the crop belong to the “famille,” which owns the stout ketches and topsail schooners in which the onions are shipped to England and Scotland. The “famille” also provides and finances the seamen of the onion fleet, and furnishes from its members the “equipe” or gang of nine or ten young salesmen—the “Onion Johnnys”—attached to each onion boat.
These last leave Britanny for these islands with the first of the season’s crop in late July or in August of each year. These Breton lads are racially more truly Britons than we ourselves. They are the direct descendants of that mighty exodus which settled in Brittany and Vendée ³,  from the Dorset, Devon, and Cornish coasts at the time when the heathen hordes were sweeping England with fire and sword all along the Saxon shore after the departure of the Roman legions ⁴. To many of them the French language is a foreign one, for their own is the old Breton tongue, once universal in England and still almost so in Wales. Roscoff and Morlaix men have often told the writer that in Wales they perfectly understand the Cymric tongue, and are perfectly understood by the Welsh in their own Breton. Wales, in fact, is their favourite part of Britain.
The onion-seller’s “equipe” or gang of eight or ten consists generally of five or six boys or youths and three or four elder men, who act as section leaders in separate districts. All the gang work from a common centre—some seaport where the ketch or schooner discharges, and where the bulk cargo is stored and strung on the familiar straw cunches [= ?] which hang from “Onion Johnny’s” shoulder-pole. One of the older men acts as advance agent for the equipe, and hires storage sheds and secures lodgings for the gang. Sometimes he purchases a horse and cart, which are resold at the end of the season—seldom, it may be stated, at a loss, for the Breton is a born “commercant” (bargainer), as housewives who are acquainted with his system of “Dutch auction” on their doorsteps can testify. Cardiff, Newcastle, Leith, Plymouth, and Douglas (Isle of Man) are favourite ports of the Roscoff men, and the writer has known them to dispose of sixty tons of onions in one month in the Manxland capital! In this instance the “equipe” was a small one of only half a dozen lads.
By the time their vessel arrives they are already on the spot, having come over by mail steamer from St. Malo or Le Havre to Southampton, and thence by railway. All are up long before sunrise unloading the little ship and carrying sacks of the onions, which are carried in the vessel’s hold in bulk, like coals or lime, across the quays to the storage sheds. These Breton boys know nothing of an eight-hour day nor believe in it, as each one of them has an interest in the whole industry which is personal. However young he may be his share is already calculated and secured at the great annual meeting of the “famille” when accounts are settled for the past season and wages, share-moneys, and bonuses are paid to him as a member of the “famille.” They are astir at 1 a.m. in the storage shed stringing the onions which will later appear on the shoulders of Marcel or Jules. Coffee and bread at 6 a.m. breaks their fast, and at 7 a.m. begins the days’ work—continued until five in the afternoon—of unloading the onion-boat. A halt is called about eleven o’clock for “déjeuner” (lunch), which consists of a little soup and meat at the adjacent lodgings. A simple cup of coffee serves until work is over, when a genuine hearty French dinner is ready for all—soup, beef, a fowl, vegetables, bread and cheese, and the everlastingly popular coffee. The tired and satisfied “equipe” are glad to sit around and rest for an hour smoking a “caporal” or hand-rolled zouave cigarette after their seventeen-hour day! By curfew hour—8 p.m.—all are abed and fast asleep, for, until the onions are unloaded and strung on the straw-ropes, 1 a.m. is their hour for starting the day’s toil. By the time the empty ketch hoists the Tricolour of France to the mizzen-peak and heads out of the harbour southwards for Roscoff, the gang have strung enough onions for a month’s sale and are busy in town and country around.
Good-natured, hardy, frugal, and industrious, Onion Johnny has his Sabbath Day rest, and Mass sees him regularly at the nearest Catholic church every seventh day. That afternoon is his very own, and, dressed in his best clothes, he may be seen abroad taking a sober pleasure in inspecting the local shipping or architectural “lions.” During the war he was on service to a man—either in the famous “Fusiliers Marins” (of whom 70 per cent. are Bretons) or in the Navy as loup-de-mer (Jack Tar).
His daily takings are handed over to the gang foreman to be banked, and each spring in his native Bretagne at the great meeting of the co-operative “famille” his share of the trading profits and his wages balance—a substantial total—rewards the participant in an honest if minor industry.

¹ The spelling ingan transcribes a southern-English pronunciation of onion.
² Rosyth is a town on the Firth of Forth, in Scotland.
³ Vendée is a département (administrative district) south of Brittany.
⁴ Brittany was occupied in the 5^th and 6^th centuries by Britons fleeing the Saxons.—Cf. origin and history of the names ‘Wales’ and ‘Cymru’.

45-year-old Jacques Scanff, an ‘Onion Johnny’ who had been plying his trade in Coventry since 1924—photograph published in The Coventry Evening Telegraph (Coventry, Warwickshire, England) of Saturday 20^th September 1958: