Heat oven to 400°F In ungreased 8-inch square (2-quart) glass baking dish, combine potato slices, contents of sauce packet from potatoes, margarine and boiling water; mix well. Stir in milk. Bake at 400°F for 20 minutes.
Meanwhile, cook ground beef and onion in large skillet over medium-high heat until beef is thoroughly cooked and onion is tender, stirring frequently. Drain if necessary. Stir in ketchup and pepper. Keep warm.
Remove baking dish from oven. Stir potatoes gently. Spoon meat mixture over potatoes. Top with tomato slices. Sprinkle with cheese.
Return to oven; bake an additional 15 to 20 minutes or until potatoes are tender and cheese is melted. Let stand 5 minutes before serving to thicken sauce.