La Stupenda is the nickname that the Venetian opera audience gave to Joan Sutherland ¹ when she sang Alcina ², an opera by Handel ³, at the Fenice Theatre ⁴ on Sunday 21^st February 1960. The Italian è stupenda translates as she is stupendous.

¹ Joan Sutherland (1926-2010) was an Australian operatic soprano.
² Alcina was first produced in 1735.
³ George Frideric Handel (1685-1759) was a German-born British composer and organist.
⁴ The Italian feminine noun fenice translates as phoenix.

(Cf. also the Emmaville Express, the nickname given to the Australian sprinter Debbie Wells.)

The earliest occurrences of La Stupenda that I have found are from the Daily Express (London, England) of Monday 22^nd February 1960:

The Critics
Noel Goodwin flies to Italy and hears them call Joan Sutherland
LA STUPENDA

Venice, Sunday.
Pink carnations which decorated the boxes and galleries of the graceful Fenice Theatre, one of Europe’s loveliest opera houses, were showered last night on the woman the audience cheered as “La Stupenda.”
She was Joan Sutherland, Covent Garden’s star soprano, enjoying a triumph that ended with the management giving her a contract to return next year in two more operas.
Last night the Australian-born redhead sang Handel’s “Alcina,” specially produced for her, and the first time the opera has been staged in Italy.
Venetians around me, considered one of the chilliest opera audiences in Italy, purred with delight at her radiant singing.
[…]
It was Mr. Zeffirelli’s ⁵ production of “Lucia” at Covent Garden a year ago which first launched Miss Sutherland into the international class of opera singers.
But with “Alcina,” his first major production of an eighteenth-century baroque opera, the producer had to convince Italians that Handel was worth seeing as well as hearing.
He did so by staging it as though it was an entertainment at some ornate ducal palace two centuries ago. The setting was a huge ballroom hung with chandeliers. The chorus acted as the guests.
The story of the beautiful temptress Alcina and her downfall, which Handel composed for Covent Garden in 1735, was then played out on a small revolving dais within the ballroom scene.

⁵ Franco Zeffirelli (born Gianfranco Corsi – 1923-2019) was an Italian film and theatre director.

The second-earliest occurrence of La Stupenda that I have found is from the column Music Chat, published in the Minneapolis Morning Tribune (Minneapolis, Minnesota) of Sunday 19^th June 1960:

A new lyric soprano hailed as “la stupenda” in Venice and lauded to the skies at the Glyndebourne festival in England is Joan Sutherland, Australian-born singer now in the Covent Garden company.

This is the beginning of an article about Joan Sutherland, by Jean Hogan, published in the Evening Express (Aberdeen, Aberdeenshire, Scotland) of Monday 1^st August 1960:

There is only one absolute and shining confirmation that you are a prima donna of international importance—and it arrived by post recently for Miss Joan Sutherland. A letter asking her to sing at La Scala, Milan, for two months, April and May, next year.
“La Stupenda,” as the Italians call her, has now, in the view a Milan newspaper (not a safe place to air such views unless they happen to be generally regarded as true) “surpassed Maria Callas ⁶ as the world’s leading soprano.”
These two gorgeous tributes plop into the placid pool of Miss Sutherland’s imperturbable personality like small aimless pebbles.
She tries, with polite energy, to say things like “I’m thrilled, thrilled, thrilled,” but one knows that, indefinably, she is feeling something else.
Backstage superlatives are both too big and too small for her. Not simply because she is a large-boned, straightforward girl from Australia, where directness is a national output. But because her voice, that surprising, brilliant voice, sits on her shoulder like a magical bird.

⁶ Maria Callas (Maria Cecilia Anna Kalageropoulos – 1923-1977) was a U.S.-born operatic soprano, of Greek parentage.

Joan Sutherland—photograph published in The Australian Women’s Weekly (Sydney, New South Wales) of Wednesday 31^st July 1974: