Wash the potatoes and brush the skin with a brush. Possibly halve or quarter the potatoes. Cook in boiling salted water for about 20 minutes. Clean, wash and halve the mushrooms.
Peel and finely chop the onion. Heat 10 g fat in a pot. Sauté the mushrooms in it and take them out. Heat 40 g fat, sauté onions, add flour and sauté. Deglaze with milk and stock while stirring and bring to the boil.
Season to taste with salt, pepper and nutmeg. Drain the potatoes and put them into an ovenproof casserole dish. Add the mushrooms and sauce and bake in a preheated oven (electric oven: 200 °C/ gas: level 3) for approx. 10 minutes.
Clean and wash the sugar snap peas and cook in a little salted water for about 5 minutes. Drain on a sieve. Melt the remaining fat and toss the sugar snap peas in it and sprinkle with sesame seeds. Serve the béchamel potatoes, sugar peas and roast beef together.