COOK onions in large nonstick ovenproof skillet on medium heat 3 to 4 min. or until crisp-tender. Stir in corn and peppers; cover. Cook 5 to 6 min. or until corn is tender, stirring frequently.
MIX flour and milk until blended. Add to corn mixture; mix well. Cook 4 to 5 min. or until thickened, stirring frequently. Add zucchini; mix well. Top with cheese.
HEAT broiler. Broil vegetable mixture 3 to 4 min. or until cheese is melted.