Meanwhile, cook mushrooms in large skillet sprayed with cooking spray on medium heat 3 to 5 min. or until tender, stirring frequently. Drain mushrooms; discard liquid. Return mushrooms to skillet. Add sausage and onions; cook 6 to 8 min. or until sausage is done, stirring occasionally. Stir in sauce; cook 5 min. or until heated through, stirring occasionally.