Heat oven to 375°F. Grease pie plate, 10x1-1/2 inches. Cook and stir ground beef and onion in 10-inch skillet over medium heat until beef is brown; drain. Stir in salt and pepper. Spread in plate. Beat milk, eggs and 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 and cheese. Bake until knife inserted in center comes out clean, 5 to 8 minutes. Cool 4 minutes.
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