HEAT oven to 375°F.
MIX sour cream, 1-1/2 cups cheese, soup and onions in large bowl until blended. Add potatoes; mix well.
SPOON into 13x9-inch baking dish sprayed with cooking spray; sprinkle with remaining cheese.
BAKE 45 to 50 min. or until casserole is heated through and golden brown on top. Let stand 5 min. before serving.