Pour cold water over buckwheat groats and drain on a sieve. Peel and finely dice the onion. Heat butter in a pot. Fry the onion and soup vegetables in it.
Add buckwheat and stock. Bring everything to the boil and simmer for about 20 minutes. Season with salt and pepper. Let buckwheat cool down and stir in the egg. Heat the oil in portions in a pan.
Bake small meatballs from the buckwheat mixture. Cut the cress from the bed. Mix crème légère and cress. Clean and wash lettuce leaves and tomatoes. Arrange buckwheat meatballs with lettuce and tomatoes on plates.
Place 1 blob of crème légère on each and garnish with parsley.