HEAT oven to 400°F. In ungreased 8x8-inch glass baking dish, combine potato slices, cheese sauce packet, butter and boiling water. Mix well. Stir in milk. Bake 20 minutes.
COOK ground beef and onion in large skillet over medium-high heat, stirring frequently, until beef is browned and onion is tender. Drain. Stir in ketchup and pepper.
REMOVE baking dish from oven. Stir potatoes gently. Spoon meat mixture over potatoes. Top with tomato slices. Sprinkle with cheese.
BAKE an additional 15 minutes or until potatoes are tender and cheese is melted. Let stand 5 minutes before serving to thicken sauce.