Question: Stefan Janski is an opera director, he received significant acclaim for his production of which of Handel's opera seria in three acts, first performed in London on 15 April 1738?

Context articles: Il Bellerofonte
Riccardo Primo
Radamisto (Handel)
Ezio (Handel)
Imeneo
Giulio Cesare
Serse
Siroe
Flavio
Stefan Janski

Il Bellerofonte is an 18th-century Italian opera in three acts by the Czech composer Josef Mysliveček.
 It conforms to the serious type (opera seria) that was typically set in the distant past.
 The libretto, based on the Greek legend of Bellerophon, was written by Giuseppe Bonecchi.
 The work was dedicated to King Ferdinand I of the Two Sicilies and was first performed at the Teatro San Carlo in Naples on 20 January 1767, the birthday of his father, King Charles III of Spain.
 The cast featured two stellar singers of the time, Caterina Gabrielli and Anton Raaff, in the leading roles.
 The opera was only the composer's second one, and the first that permitted him the opportunity to write music for first-rate vocal artists.
 The production was highly successful, indeed responsible for a meteoric rise in his reputation as an operatic composer.
 From the time of the premiere of "Bellerofonte" until his death in 1781, Mysliveček succeeded in having more new "opere serie" brought into production than any other composer in Europe.
 During the same time span, he also had more new operas staged at the Teatro San Carlo in Naples than any other composer.
Riccardo primo, re d'Inghilterra ("Richard the First, King of England", HWV 23) is an opera seria in three acts written by George Frideric Handel for the Royal Academy of Music (1719) .
 The Italian-language libretto was by Paolo Antonio Rolli, after Francesco Briani's "Isacio tiranno", set by Antonio Lotti in 1710.
 Handel wrote the work for the Royal Academy's 1726–27 opera season, and also as homage to the newly crowned George II and the nation where Handel had just received citizenship.
Radamisto (HWV 12) is an opera seria in three acts by George Frideric Handel to an Italian libretto by Nicola Francesco Haym, based on "L'amor tirannico, o Zenobia" by Domenico Lalli and "Zenobia" by Matteo Noris.
 It was Handel's first opera for the Royal Academy of Music.
 The opera's plot is loosely based on incidents from Tacitus's "Annals of Imperial Rome".
Ezio ("Aetius", HWV 29) is an opera seria by George Frideric Handel to a libretto by Metastasio.
 Metastasio's libretto was partly inspired by Jean Racine's play "Britannicus".
 The same libretto had already been set by many other composers first of all Nicola Porpora who managed to preempt the official Rome premiere of Pietro Auletta's setting for 26 December 1728 with his own version (of a slightly edited copy of the libretto) for Venice on 20 November, a month earlier.
 The libretto continued to be set and reset for another 50 years, including two versions of "Ezio" by Gluck.
 Handel's "Ezio" is considered one of the purest examples of opera seria with its absence of vocal ensembles.
Imeneo (alternative title: Hymen, HWV 41) is an opera seria in three acts by George Frideric Handel.
 The Italian-language libretto was adapted from Silvio Stampiglia's "Imeneo".
 Handel had begun composition in September 1738, but did not complete the score until 1740.
 The opera received its first performance at the Lincoln's Inn Fields in London on 22 November 1740, and received another performance on 13 December.
 Handel then revised the score, and this revised version received concert performances in Dublin, on 24 and 31 March 1742.
Giulio Cesare in Egitto (] ; Italian for "Julius Caesar in Egypt", HWV 17), commonly known as Giulio Cesare, is a dramma per musica ("opera seria") in three acts composed for the Royal Academy of Music by George Frideric Handel in 1724.
 The libretto was written by Nicola Francesco Haym who used an earlier libretto by Giacomo Francesco Bussani, which had been set to music by Antonio Sartorio (1676).
 The opera was a success at its first performances, was frequently revived by Handel in his subsequent opera seasons and is now one of the most often performed Baroque operas.
Serse (] ; English title: "Xerxes"; HWV 40) is an opera seria in three acts by George Frideric Handel.
 It was first performed in London on 15 April 1738.
 The Italian libretto was adapted by an unknown hand from that by Silvio Stampiglia for an earlier opera of the same name by Giovanni Bononcini in 1694.
 Stampiglia's libretto was itself based on one by Nicolò Minato that was set by Francesco Cavalli in 1654.
 The opera is set in Persia (modern-day Iran) in 480 BC and is very loosely based upon Xerxes I of Persia, though there is little in either the libretto or music that is relevant to that setting.
 Serse, originally sung by a soprano castrato, is now usually performed by a soprano (or mezzo-soprano) and sopranist.
Siroe, re di Persia ("Siroes, King of Persia", HWV 24), is an opera seria in three acts by George Frideric Handel.
 It was his 12th opera for the Royal Academy of Music and was written for the sopranos Francesca Cuzzoni and Faustina Bordoni.
 The opera uses an Italian-language libretto by Nicola Francesco Haym, after Metastasio's "Siroe".
 Like many of Metastasio's libretti, it was also set by Handel's contemporaries, e.g. by Leonardo Vinci, Antonio Vivaldi and Johann Adolph Hasse (see Hasse's "Siroe").
 Pasquale Errichelli's setting of the libretto (see Errichelli's "Siroe") premiered in the year of Handel's death.
Flavio, re de' Longobardi ("Flavio, King of the Lombards", HWV 16) is an opera seria in three acts by George Frideric Handel.
 The Italian-language libretto was by Nicola Francesco Haym, after Matteo Noris's "Flavio Cuniberto".
 It was Handel's fourth full-length opera for the Royal Academy of Music.
 Handel had originally entitled the opera after the character of Emilia in the opera.
Stefan Janski is an opera director.
 He was the co-producer and librettist for the Sadler’s Wells Theatre production of "The Snowman of Kashmir" in 1974, and has served as Associate Director with Glyndebourne Festival Opera and its Touring Opera, as well as staff director for English National Opera.
 He is Director of Opera Studies at the Royal Northern College of Music, which he joined in 1986.
 He has also worked freelance for notable companies such as Scottish Opera, Los Angeles Opera and the Royal Ballet of Flanders.
 He received significant acclaim for his production of Handel's "Xerxes".