New-Look Scalloped Potatoes and Ham

4-1/2 lb. red potatoes, cut into 1/4-inch-thick slices
1 container (16 oz.) BREAKSTONE’S FREE or KNUDSEN FREE Fat Free Sour Cream
3/4 lb. (12 oz.) VELVEETA LIGHT Reduced Fat Pasteurized Prepared Cheese Product, cut up
1 pkg. (8 oz.) OSCAR MAYER Smoked Ham, chopped
1/2 cup green onion slices
1/4 cup KRAFT 100% Grated Parmesan Cheese

PREHEAT oven to 350°F. Spray 13x9-inch baking dish with cooking spray. Place potatoes in large saucepan; cover with water. Bring to boil on high heat. Reduce heat to low; cover and simmer 10 to 12 min. or until tender. Drain potatoes. Return potatoes to saucepan.
TRANSFER 1/4 of the potatoes to large bowl. Add sour cream; mash with potato masher. Add Velveeta, ham and green onions; mix lightly. Add mashed potato mixture to potato slices in saucepan; stir gently. Spoon into prepared baking dish. Sprinkle with Parmesan cheese.
BAKE 30 min. or until heated through.