Peel, wash and chop the potatoes and carrots. Cover and cook in salted water for about 20 minutes
Clean the mushrooms, wash if necessary and dab dry. If necessary cut a little smaller. Peel and finely chop the onion. Wash and chop the parsley. Fry the mushrooms in hot oil in portions. Fry the onion briefly. Stir in parsley. Season with salt and pepper
Drain potatoes and carrots and mash them coarsely, while stirring in 2 tablespoons of butter and 1/8 l milk. Season puree with salt, pepper and nutmeg
Grease the casserole dish (approx. 30 cm long). Spread the puree in it, press a depression in the middle. Spread the mushrooms in it and press lightly. Mix egg and 5 tablespoons of milk. Season with salt and pepper. Pour over the mushrooms. Grate cheese over them
Bake in a preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: level 3) for 15-20 minutes. Garnish with the remaining parsley. Black Forest Ham goes well with it
Drink: cool mineral water or dry cider