HEAT oven to 350°F.
COOK potatoes in boiling water in Dutch oven or stockpot 10 to 12 min. or until tender; drain.
PLACE 1/4 of the potatoes in large bowl. Add sour cream; mash until smooth. Add Cheez Whiz, ham and onions; mix lightly. Gently stir in remaining potato slices. Spoon into 13x9-inch baking dish sprayed with cooking spray; top with Parmesan.
BAKE 30 min. or until heated through.
serving size = 1/16th recipe (236 g)