Wash the potatoes and precook for about 15 minutes. Clean the mushrooms, wash if necessary and drain. Fry the mushrooms in 1 tbsp. hot fat for about 8 minutes. Season with salt and pepper
Sweat the flour in 2 tbsp. hot fat. Stir in 1/4 l water, milk and stock, bring to the boil. Simmer for about 5 minutes. Season sauce with salt, pepper and nutmeg
Grate the cheese roughly. Quench potatoes, peel them and spread them with mushrooms in a greased casserole dish. Spread sauce and cheese over it. Bake in a hot oven (electric cooker: 200 °C/ convection oven: 175 °C/gas: level 3) for approx. 20 minutes