Peel and chop the onion. Clean, peel and wash the kohlrabi. Grate some kohlrabi finely to garnish, dice the rest roughly. Peel, wash and chop the potatoes.
Heat the oil in a large pot. Sauté the onion in it until transparent. Add diced kohlrabi and potatoes and fry. Pour on 1/2 l water, bring to the boil and stir in stock. Cover and simmer at medium heat for about 20 minutes.
Cut salmon into strips. Finely puree the soup with a hand blender. Stir in sour cream. Season to taste with salt, pepper and nutmeg. Serve soup with salmon strips and kohlrabi shavings.