HEAT sauce according to package directions. Spread a little sauce on bottom of 9 in. (23 cm) baking dish.
LAYER 1/2 the potatoes, ham and onions in dish. Pour 1/2 the jar of sauce over layers. Sprinkle with half the Parmesan cheese. Repeat all layers.
COVER and bake at 350°F (180°C) for 20 minutes. Remove cover and bake 10 minutes longer until lightly browned.