For the celery soup with trout fillet, heat the butter in a pot. Cut the onion and the beetroot into small cubes and fry them in the butter.
Divide the celery into larger pieces, add them, let them brown a bit and pour water on them. Add soup powder, salt, pepper and nutmeg and simmer for about 15 minutes. Then puree everything finely with a hand blender.
Divide the trout fillet into pieces, distribute on the plates, pour hot soup and serve the celery soup with trout fillet.