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.