Preheat oven to 450°.
Peel onions, leaving roots intact; cut each onion into 6 wedges.
Place onion wedges in an 11 x 7-inch baking dish coated with cooking spray. Combine vinegar and remaining ingredients in a small bowl; pour vinegar mixture over onion wedges, tossing gently to coat. Cover and bake at 450° for 25 minutes. Uncover and bake an additional 45 minutes or until tender.