HEAT oven to 350°F.

COOK potatoes in boiling water in Dutch oven 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.