Cook the potatoes with their skins in boiling salted water for about 20 minutes. Cut the kohlrabi into sticks and cook in salted water for about 10 minutes.
Melt the margarine in a saucepan, dust with flour and stir. Add milk and stir with a whisk to a smooth sauce. Melt the cheese in the sauce, stir in the parsley and season with salt, pepper and nutmeg.
Peel the potatoes hot, arrange on plates with the ham, serve with kohlrabi and sauce.