Wash the chicken breasts and boil them in 1 litre of salted water. Peel the onion, add to the chicken with the bay leaf and peppercorns. Cook everything over medium heat for about 30 minutes. Peel, wash and cut the kohlrabi and carrots into small pieces.
Remove the chicken breasts and let them cool down a little. Pour broth onto a sieve, collect the liquid and bring to the boil again. Cook the vegetables for about 15 minutes. Add peas to the vegetables about 4 minutes before the end of the cooking time.
Put the rice in plenty of boiling salted water and cook for about 20 minutes. Remove chicken meat from skin and bone and cut into cubes. Pour vegetables onto a sieve, collect the liquid and measure 1/4 litre.
Melt the fat in a pot. Sweat flour in it. Deglaze with vegetable stock and milk while stirring. Bring to the boil and simmer for about 3 minutes. Add vegetables and chicken meat to the sauce. Season to taste with salt, pepper and lemon juice.
Wash the chervil, dab dry and, except for something to garnish, chop coarsely and add to the ragout. Pour the rice onto a sieve and drain. Arrange chicken ragout and rice on plates. Serve garnished with lemon and chervil.