Rinse chicken fillets, dab dry, season with salt and pepper. Fry in hot oil for about 5 minutes on each side. Take out, let cool down a little. In the meantime, wash and clean the sugar snap peas and blanch in boiling salted water for about 1 minute.
Rinse with cold water and let it drain. Clean and peel the carrots and kohlrabi. Cut carrots diagonally into thin slices. Cut kohlrabi into thin slices, halve or quarter them depending on size.
Lightly grease the casserole dish and layer the vegetables in it. Cut the chicken fillets into thick slices and put them in the middle. For the béchamel sauce, peel and finely chop the onion. Melt the fat in a pot and fry the onion until transparent.
Dust with flour, sweat, gradually add stock and milk. Simmer for 5 minutes, stirring from time to time. Stir in processed cheese, season to taste with salt and pepper. Pour hot over the casserole.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-25 minutes. Garnish with chervil if desired and serve immediately.