Impossible Tuna Cheese Pie

2 medium Onions -- Chopped

1/4 cup Butter or Margarine

2 cans Tuna -- Drained 6-1/2oz

2 cups Shredded Cheddar Cheese

1 cup Biscuick

1 1/4 cups Milk

1/4 teaspoon Salt -- If Desired

1/8 teaspoon Pepper

3 Eggs

2 Tomatoes -- Thinly Sliced

Heat oven to 400~. Grease 8x8x2" square baking dish. Cook onions and margarine in 10" skillet over low heat, stirring occasionally, until onions are light brown. Sprinkle tuna, 1 cup of the cheese and the onions in plate. Beat Bisquick, milk, salt, pepper and eggs with wire whisk or hand beater until almost smooth, about 1 minute. Pour into plate. Bake until knife inserted in center comes out clean, 25 to 30 minutes. Top with tomato slices and remaining cheese. Bake until cheese is melted 3 to 5 minutes.