For the vegetable stock, clean or peel, wash and cut the soup greens into large pieces. Wash and halve the tomato. Peel and halve 1 onion. Heat the oil in a large pot. Brown the onion on the cut surfaces.
Add the soup greens and continue roasting for about 3 minutes. Add tomato. Deglaze with 1 1⁄2 l water. Add bay leaf, 1 1⁄2 tsp. salt and peppercorns. Bring to the boil, cover and simmer over low heat for about 45 minutes.
Wash the meat, dab dry. Add to the vegetable stock after about 30 minutes. Wash parsley, shake dry and chop leaves finely. Grate cheese and bread finely. Mix parsley, cheese and bread.
Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Lift chicken fillets out of the broth and let them cool down for about 15 minutes. Pour stock through a sieve and collect. Measure out 300 ml stock.
Use the remaining stock for other purposes or freeze it.
Peel and finely dice 1 onion. Heat 3 tablespoons of butter in a saucepan. Fry the onion until transparent. Dust with flour and briefly sauté lightly while stirring. Add measured broth while stirring, bring to the boil and simmer for about 5 minutes.
Stir in mustard and crème fraiche. Season to taste with salt, pepper and sugar.
Roughly pluck the chicken fillets into pieces, place in an ovenproof dish and spread the sauce over them. Sprinkle with the bread and cheese mixture. Spread 4 tablespoons of butter in flakes on top and bake in a hot oven for 20-25 minutes.