In a skillet, brown ground beef with onion; drain. Stir in spaghetti sauce, seasonings, water and mushrooms; mix well. Pour half into a 13x9-in. baking dish. Cover with potatoes. Pour remaining sauce over potatoes. Cover tightly with foil.
Bake at 350° for 1 hour and 15 minutes or until potatoes are tender. Sprinkle with mozzarella cheese; return to oven for another 10 minutes or until the cheese is melted.