Peel, rinse and slice the potatoes.
Rinse and slice the zucchini. Rinse and slice the mushrooms.
Peel the onions and cut into rings.
Alternately layer the prepared ingredients in a greased baking dish. Season with salt, pepper and nutmeg. Sprinkle with parsley. Whisk the egg yolks and cream. Fold in the beaten egg whites. Pour the mixture over the vegetables. Sprinkle the cheese over the casserole and top with butter flakes.
Bake for about 40 minutes at 180°C (approximately 350°F), until golden brown. Remove from the oven and serve immediately.