Heat oven to 400 degrees.Grease square baking dish,8 x 8 x 2". 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 dish. Beat baking mix,milk,pepper and eggs with wire whisk or hand beater until almost smooth,about 1 minute.Pour into dish. 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.Cool 5 minutes.Serves 6 to 8.