Wash the potatoes and scrub the skin with a brush, possibly halve or quarter it. Pour into boiling salted water and cook for 20 minutes. Clean, wash and halve the mushrooms.
Peel and finely chop the onion. Heat 10 g fat in a pot. Brown the mushrooms in it and take them out. Heat remaining fat, fry onions, add flour and sauté in it. Deglaze with milk and stock while stirring and bring to the boil.
Season to taste with salt, pepper and nutmeg. Drain the potatoes and add to the sauce with the mushrooms. Put them into an ovenproof casserole dish and bake in a preheated oven (electric oven: 200 °C/ gas: level 3) for 10 minutes.