Place the flour in hot margarine or butter, stir, pour in the milk and cook for about 15 minutes, constantly stirring. Then add the cheese diced in larger cubes, pepper, mustard, salt, and cook again until the cheese melts evenly. Finally stir in the finely sliced hard-boiled egg and parsley. “Champignon” sauce can be served with a variety of roast meat dishes or with grilled steaks.