Question: Which opera in two acts by Wolfgang Amadeus Mozart did Franz Xaver Gerl sang the role of Sarastro in the premiere of

Context paragraphs:
Franz Xaver Süssmayr: Franz Xaver Süssmayr (German: "Franz Xaver Süßmayr" or "Suessmayr" in English; 1766 – September 17, 1803) was an Austrian composer and conductor.  Popular in his day, he is now known for his completion of Wolfgang Amadeus Mozart’s Requiem in D Minor.  In addition, there have been performances of Süssmayr’s operas at Kremsmünster, and a new version of his cantata "Der Retter in Gefahr" was performed.  There are also CD recordings of his unfinished clarinet concerto (completed by Michael Freyhan), one of his German requiems, and his Missa Solemnis in D.

Serenade No. 7 (Mozart): The serenade for orchestra in D major, K. 250 (248b), popularly known as the Haffner Serenade, is a serenade by Wolfgang Amadeus Mozart named for the Haffner family.  Mozart's friend and contemporary Sigmund Haffner the Younger commissioned the serenade to be used in the course of the festivities before the wedding of his sister Marie Elisabeth Haffner and her intended, Franz Xaver Spaeth.  The Serenade was first played on 21 July 1776, on the eve of the wedding.  It is in eight movements:

Requiem (Mozart): The Requiem in D minor, K. 626, is a requiem mass by Wolfgang Amadeus Mozart.  Mozart composed part of the "Requiem" in Vienna in late 1791, but it was unfinished at his death on 5 December the same year.  A completed version dated 1792 by Franz Xaver Süssmayr was delivered to Count Franz von Walsegg, who commissioned the piece for a Requiem service to commemorate the anniversary of his wife's death on 14 February.

Der Stein der Weisen: Der Stein der Weisen, oder die Zauberinsel (English: The Philosopher's Stone, or the Enchanted Isle) is a two-act singspiel jointly composed by , Benedikt Schack, Franz Xaver Gerl, Emanuel Schikaneder, and Wolfgang Amadeus Mozart in 1790 (K. 592a).  The libretto was written by Schikaneder.

The Magic Flute: The Magic Flute (German: Die Zauberflöte ), K. 620, is an opera in two acts by Wolfgang Amadeus Mozart to a German libretto by Emanuel Schikaneder.  The work is in the form of a "Singspiel", a popular form that included both singing and spoken dialogue.  The work premiered on 30 September 1791 at Schikaneder's theatre, the Freihaus-Theater auf der Wieden in Vienna, just two months before the composer's premature death.

Amadeus (film): Amadeus is a 1984 American period drama film directed by Miloš Forman, adapted by Peter Shaffer from his stage play "Amadeus".  The story, set in Vienna, Austria, during the latter half of the 18th century, is a fictionalized biography of Wolfgang Amadeus Mozart.  Mozart's music is heard extensively in the soundtrack of the movie.  The film follows Italian composer Antonio Salieri at the court of Emperor Joseph II, and his jealous vendetta against his younger rival, Wolfgang Amadeus Mozart.

Franz Xaver Wolfgang Mozart: Franz Xaver Wolfgang Mozart (26 July 1791 – 29 July 1844), also known as Wolfgang Amadeus Mozart, Jr., was the youngest child of six born to Wolfgang Amadeus Mozart and his wife Constanze.  He was the younger of his parents' two surviving children.  He was a composer, pianist, conductor, and teacher from the late classical period whose musical style was of an early Romanticism, heavily influenced by his father's mature style.

Franz Xaver Gerl: Franz Xaver Gerl ((1764--)30 1764 – (1827--)09 1827 ) was a bass singer and composer of the classical era.  He sang the role of Sarastro in the premiere of Mozart's opera "The Magic Flute".

Franz Xaver Niemetschek: Franz Xaver Niemetschek (; Polish: "Niemeczek" ) (24 July 1766 – 19 March 1849) was a Czech philosopher, teacher and music critic.  He wrote the first full-length biography of Wolfgang Amadeus Mozart which has remained an important source of information about the composer.

Constanze Mozart: Maria Constanze Cäcilia Josepha Johanna Aloysia Mozart (née Weber) (5 January 1762 – 6 March 1842) was an Austrian woman who trained as a singer.  She married twice, her first husband being Wolfgang Amadeus Mozart, and was later, jointly with her second husband Georg Nikolaus von Nissen, Mozart's biographer.  She and Mozart had six children, Karl Thomas Mozart, Franz Xaver Wolfgang Mozart, and four who died in infancy.

Supporting paragraphs: Franz Xaver Gerl The Magic Flute
Answer: The Magic Flute