Chop fine the onion. Cook it, over hot water, in butter. When the onion is soft add mushrooms, flour and stock. Stir until boiling. Add chopped parsley, salt and pepper. Put a tablespoonful of this sauce in the bottom of individual cups. Break into each cup one egg. Pour over the remaining mixture. Stand the cups in a pan of hot water and bake in a moderate oven about five minutes.