Cook vegetables in boiling water in medium saucepan 3 min. or until crisp-tender; drain. Place in 13x9-inch baking dish sprayed with cooking spray.
Whisk dressing, flour and garlic powder in same saucepan until blended. Gradually stir in milk; cook on medium heat 3 to 5 min. or until sauce comes to boil and is thickened, stirring frequently. Stir in Parmesan; pour over vegetables. Top with shredded cheese.
Prepare stuffing as directed on package; spoon over cheese.
Bake 35 min. or until heated through.