Clean, peel and cut the kohlrabi and potatoes into cubes. Finely chop the onion and garlic.
Froth the butter in a saucepan. Fry the onion and garlic in it, add the vegetables and season with salt and pepper. Fry for 3 minutes. Add the stock and simmer covered for 10 minutes. Then remove the lid and let it boil down.
Whip the cream until it is half stiff and mix in with the rosemary. Bring to the boil briefly and then stir in the parmesan. Season to taste with salt and pepper.