Clean and wash the radishes and cut them into fine slices. Peel and finely chop the onion. Season vinegar with salt, pepper and sugar. Stir in oil and diced onion. Marinate the radish slices with the oil and leave to stand.
In the meantime clean, wash and cut the kohlrabi into thin slices. Blanch in boiling salted water for about 3 minutes, then drain. Wash the chives and cut into small rolls. Arrange kohlrabi and radishes with the marinade on salad.
Put a dollop of cream on each. Serve sprinkled with chervil leaves and chives.