Tuna Cheese Pie

1 medium onion
2 cans tuna, drained
2 cups shredded cheese
1/4 tsp salt
3 eggs
1/4 cup butter
1 cup Bisquick
1 1/4 cups milk
1/8 tsp pepper
2 tomatoes, thinnly sliced

Heat oven to 400 degrees. Grease square baking dish. Cook onions in oil over low heat until light brown. Sprinkle tuna, one cup of cheese, and onions in dish. Beat Bisquick, milk, salt, pepper, and eggs with whisk or hand beater one minute. Pour into dish. Bake until knife inserted comes out clean. About 20 minutes. Add tomatoes, spread on cheese.