HEAT oven to 450ºF.
RESERVE 2 Tbsp. dressing for later use. Place fish on baking sheet sprayed with cooking spray; brush with remaining dressing.
BAKE 12 to 15 min. or until fish flakes easily with fork. Meanwhile, heat oil in large nonstick skillet on medium heat. Add next 3 ingredients; cook and stir 4 min. or until crisp-tender. Remove from heat; stir in reserved dressing.
SPOON cooked vegetables onto serving plate; top with fish and onions.