Hi Anne, welcome !!
Mozart said the organ is the king of instruments. He wrote many works for it, as well as he worked as an organist in Saltzburg. In December 1769 at the age of 13 he did a recital in Verona at San Tommaso's organ where he played the Veronese Allegro, probably written for the occasion.
There's a painting by Heinrich Lossow of 1864 with Mozart as a kid on a pipe organ console and his legs not reaching the pedals of course
I'll try to scan and post here.
Mechanical organs with clockwork mechanism were very popular in Vienna at the time (became a rage!!), because people could afford some pipe organ sound at home. There was a company who asked Mozart to compose some music for their models and indeed the pieces linked here by Lars are some of them. Though the Adagio & Allegro in F minor K594 is more suited for funerals..... khmm
Famous are the Church sonatas for two violins, bass and organ, where Mozart wrote on score "only copulas" for registration (ie Copula 8+4) as the mild flute tones mix well with strings. My favourite is the F major K244 one.
These sonatas can also be played with a wider string orchestra and positiv organ as well as originally written for the small ensemble.
I got two performances to offer : Marie-Claire Alain's on Haerpfer-Ermann organ at St.Nicolas in Haguenau (Bas-Rhin) with Chamber orchestra of Jean-Francois Paillard in 1972.
And Herbert Tachezi's at the organ of the Basilica Maria Treu (Pianistenkirche) Vienna, with Alice Harnoncourt & Walter Pfeiffer violins and Nikolaus Harnoncourt cello in 1969.
There is also Fugue in G minor KV401 (KV 357e) for big organ (with Thomas Trotter is a good example) as well as the beautiful Andante in F KV611 (good with the two copulas on...), the Andante in F K616, the Allegro in G major K72a and Gigue in G major K574.
Daniel Chorzempa did all the sonatas and the other solo pieces for Phillips too, part of Mozart complete bundle.
Good option for playin' Mozart on organ are the choir, positiv and/or continuo organs, as they can be placed not only in churches but in halls too, thus giving more options to the performer and the inclusion of a chamber orchestra. At Mozart's time there were many positiv organs in Palaces ....
So, as you see there's a lot to be played... :-D
Have fun !
Best
Panos