Fry the prepared chicken fillets in a little oil for about 5 minutes on each side. Take out of the pan and let rest.
Simmer potatoes, carrots and kohlrabi in lightly salted water for about 5 minutes, lift out and pour into a greased baking dish, spread the peas on top. Save the cooking water for the sauce. Cut the chicken fillet into slices and spread over the vegetables.
For the sauce, sweat the onion cubes in the butter, do not brown. Dust with the flour, sweat briefly and pour 300 ml of cooking liquid (add water or milk to the required amount if necessary), season well with salt and pepper and add chopped parsley if desired. If you like, you can stir in some cream cheese.
Pour the sauce over the casserole, sprinkle with grated cheese or some Parmesan if you like and place in the oven at 180 ° C (top / bottom heat 200 ° C) for approx. 30 - 40 minutes until the vegetables are cooked.
The cooking time depends somewhat on the size of the vegetable pieces and on whether you are using very young vegetables.