Peel and finely dice the onion. Knead minced meat, onion, mustard, egg, breadcrumbs and 4 tablespoons of milk. Season with salt and pepper. Shape into 4 meatballs. Prepare rice in boiling salted water according to package instructions.
Heat the oil in a pan and fry the meatballs for 10-15 minutes while turning. Peel and wash the carrots and cut them lengthwise into quarters. Peel and halve kohlrabi and cut into slices. Cook prepared vegetables in boiling salted water for about 6 minutes.
Melt the fat in a pot. Stir in flour and sweat briefly. Add stock and 1/8 l milk while stirring. Bring to the boil, boil for 3-5 minutes and season with salt and pepper. Wash the chervil, shake dry, pluck off the leaves, except for a little to garnish.
Pour the vegetables into a sieve, add to the sauce and fold in. Arrange rice, vegetables and meatballs on plates. Sprinkle with chervil and garnish.