Coat a large nonstick skillet with cooking spray; place over medium-high heat. Place 1 tortilla in skillet, and top with 1/2 cup cheese, 2 tablespoons onions, and another tortilla. Cook 2 minutes on each side or until lightly browned. Remove from skillet, and keep warm. Repeat with remaining tortillas, cheese, and onions. Cut each quesadilla into 6 wedges.