HEAT oven to 425ºF.
COOK fries as directed on package.
MEANWHILE, brown meat with onions in large skillet; drain. Add mushrooms; cook 4 to 5 min. or until tender. Stir in sauce; cook 3 to 4 min. or until heated through, stirring occasionally.
TRANSFER fries to baking dish; top with meat sauce and cheese. Bake 2 min. or until cheese is melted.