Wash the potatoes and cook for about 15 minutes. Clean the mushrooms, wash them if necessary and drain them. Heat 2 tablespoons of fat in a large pan. Fry the mushrooms for about 8 minutes until golden brown. Season with salt and pepper.
Heat 5 tablespoons of fat in a saucepan. Sauté the flour in it. Deglaze with 1/2 litre water and milk. Bring to the boil, stir in broth and simmer for about 5 minutes. Season béchamel with salt, pepper and nutmeg. Grate cheese coarsely. Drain, rinse and peel the potatoes. Mix potatoes and mushrooms and put them into a greased casserole dish. Pour sauce over it.
Grate cheese coarsely. Drain, rinse and peel the potatoes. Mix potatoes and mushrooms and put them into a greased casserole dish. Pour sauce over it. Sprinkle with cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes
With 10 people: