Sigfrid Karg-Elert (November 21, 1877 – April 9, 1933)
In 1902 having returned to Leipzig, he started devoting himself to composition, primarily for the piano (encouraged by Edvard Grieg, whom he greatly admired); and in 1904 he met the Berlin publisher Carl Simon, who introduced him to the harmonium. From then on until his death he created one of the most significant and extensive catalogs of original works for this instrument. Encouraged by the organist Paul Homeyer, he reworked several of these harmonium compositions for organ, before composing his first original organ pieces, the 66 Chorale Improvisations, Op. 65 in 1909.
To see what is possible on a good harmonium have a look at this clip; https://www.youtube.com/watch?v=ZiCEEgPAMfc&list=RDZiCEEgPAMfc