Peel and finely chop the onion. Peel and wash kohlrabi and potatoes. Cut both into small pieces.
Melt the fat. Sauté onion in it. Steam kohlrabi and potatoes. Add 3/4 l water and bring to the boil. Season with stock, salt and pepper. Simmer covered for about 20 minutes.
Clean and wash the radishes and cut them into thin sticks. Wash the chervil, shake dry and chop coarsely, except for a little to garnish. Add the chervil to the soup and briefly puree everything with a hand blender.
Season with salt and pepper.
Whip the cream slightly, stir in and foam up again briefly with a hand blender. Arrange immediately and sprinkle with radish sticks and chervil leaves.