Impossible Cheeseburger Pie

1 lb. ground beef

1-1/2 C chopped onion

1/2 t salt

1/4 t pepper

1-1/2 C milk

3 eggs

3/4 C Bisquick baking mix

2 tomatoes, sliced

1 C shredded Cheddar or process American cheese

Heat oven to 400 degrees. Grease pie plate, 10x1-1/2 inches. Cook and stir baking mix until smooth, 15 seconds in blender on high or 1 minute with hand beater. Pour into plate. Bake 25 minutes. Top with tomatoes; sprinkle with cheese. Bake until knife inserted in center comes out clean, 5 to 8 minutes. Cool 5 minutes. 6-8 servings.