Grease a 9 1/2" pie dish with butter or cooking spray.
In a skillet, brown beef and onion; drain any excess fat. Add salt and pepper. Spread into the grease pie dish.
In a large bowl, beat milk, baking mix and eggs until smooth or for 15 seconds in a blender on high.
Pour the batter over the cooked beef and onions. Bake for 25 minutes or until a knife inserted comes out clean.
Top the pie with cheese followed by the tomatoes. Bake for an additional 5-8 minutes unti melted and golden. Cool for 5-8 minutes before slicing and serving.