Soak the buns. Peel and finely chop the onions. Knead minced meat, onions, mustard, egg and squeezed out roll. Season with about 1 tsp salt and about 1/2 tsp pepper. Form 4 meatballs from the minced meat mixture.
Peel and wash the vegetables. Cut carrots lengthwise in half and cut into pieces. Halve kohlrabi and cut into thin slices. Boil both covered in 1/4 l water with broth and cook for about 8 minutes.
Cook rice in 1/2 l boiling salted water according to package instructions.
Heat the oil in a frying pan. Fry the meatballs in it over medium heat for 8-10 minutes on each side.
Drain the vegetables and collect the vegetable water. Heat butter in a pot. Dust with flour and briefly sauté lightly. Deglaze with vegetable water and milk while stirring. Bring to the boil and simmer for about 5 minutes.
Season with salt, pepper, 1 pinch of sugar and lemon juice.
Heat the vegetables in the sauce. Wash chervil, shake dry, pluck off leaves and stir in. Arrange everything.