Clean, wash and cut the leek into rings. Cover and cook in 250 ml boiling salted water for 6-8 minutes. Pour onto a sieve, drain and collect the vegetable water. Measure out 150 ml.
Clean, clean and slice the mushrooms. Peel onion and cut into rings. Heat the oil in a pan and fry the onion rings. Add the mushrooms and fry for about 5 minutes, turning them until they start to brown. Season with a little salt and pepper and dust with starch. Stir-fry briefly. Add 150 ml vegetable water, crème légère and ketchup, bring to the boil and simmer for about 1 minute.
Lightly grease a high casserole dish (approx. 27 x 18 cm, 1.5 litre capacity) with oil and pour in 1/4 of the mushroom sauce. Place 4 cutlets on top and cover with a layer of sauce and half of the leek. Then add another layer of schnitzel, leek and sauce. Close with a lid or well with aluminium foil. Braise in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 2 hours. After 1 1/2 hours, remove the lid or aluminium foil and cook until open.
Wash, peel and cover the potatoes and cook them in little boiling salted water for about 20 minutes. Drain potatoes. Wash parsley, shake dry and chop finely. Sprinkle over the layered meat. Add potatoes