Brown the onions and sausage. Drain well. Add the onions, Parmesan, Swiss, Tabasco, salt and parsley. Make a batter of biscuit mix, milk and mayo. Spread half of batter in 9x9x2" greased pan. Pour in sausage mixture. Spread remaining batter on top. Mix egg yolk and water and spread on top. Bake at 400~ for 25 to 30 minutes until cake leaves edge of pan. Cool for 5 minutes before cutting into 3" squares.