Clean and clean the mushrooms. Clean, wash and cut leek into rings. Grate cheese.
Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Heat oil in a pan. Fry the escalopes briefly on each side. Season with salt, pepper and marjoram and place in a flat casserole dish.
Fry the mushrooms in hot frying fat over high heat, fry the leek briefly. Season with salt and pepper. Stir in 200 ml water, cream and stock. Bring to the boil, thicken with sauce thickener and season to taste.
Pour everything over the escalopes, sprinkle evenly with cheese. Close the mould with aluminium foil and braise in a hot oven for about 1 1/2 hours. After 1 hour remove the foil and braise open to the end.
In the meantime, peel, wash and roughly dice the potatoes. Cook in salted water for about 20 minutes. Drain. Add milk and butter, warm up and mash everything finely. Season the puree with salt and nutmeg.
Dress everything.