In 1777, Mozart was having lunch with his uncle at the Holy Cross Convent in Ausburg. Mozart played a sinfoni and Vanhall’s Violin Concerto in B. In the evening, at supper, he performed his Strasbourg concerto, a keyboard prelude and his Fischer Variations (K179). It was suggested to the deacon of the Holy Cross Conen, P. Ludig Zoschinger, a composer in his own right, that Mozart should perform in the organ style:

so I asked him to give me a theme, he didn’t want to, but one of the clergymen did. I took the theme for a walk, then in the middle of it -the fugue was in g minor – I changed it to major and came up with a very sprightly little tune, but in the same tempo, then I played the theme again, but this time assbackwards; in the end, I wondered whether I couldn’t se ths merry little thing as a theme for the fugue? – Well, I didn’t stop to inquire, I just went ahead and did it, and it fit so well as if it had been measured by Daser [a tailor in Salzburg]. The Herr Dechant [Zoschinger] was quite beside himself with joy. You have done i, that’s all I can say, I would not have believed what I just heard, what a man you are… (1)

(1) Mozart, in a letter to his father, 23-25 October 1777. Cited in: Spaethling, Robert (2000) Mozart’s Letters; Mozart’s Life. London: Faber and Faber, p.80.