Preheat broiler. Cut onions crosswise into 1/4-inch-thick slices; arrange in a single layer on a baking sheet coated with cooking spray. Coat onions lightly with cooking spray. Broil 4 minutes on each side or until onions begin to brown; cool to room temperature. Chop onions; combine with parsley, olives, vinegar, oregano, and salt. Stir in feta cheese.