Butter a 2-quart square baking dish; set aside. Cook potatoes according to package directions. Spread potatoes evenly in prepared baking dish. Sprinkle with salt and pepper. Layer in order: spinach, onion, tomato, bacon, and cheese. Bake, uncovered, in a 400 degree F oven for 15 minutes. Serve with mixture of sour cream and horseradish, if desired. Makes 6 side-dish servings.