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