Combine 1/4 cup maple syrup, teriyaki sauce, lemon juice, and garlic in a large heavy-duty, zip-top plastic bag. Add fish. Seal bag, and shake gently until fish is well coated. Marinate in refrigerator 30 minutes.
Coat one side of a 12-inch square of heavy-duty aluminum foil with cooking spray. Place onion slices on half of coated side of foil. Combine remaining 2 tablespoons maple syrup and margarine, stirring well. Drizzle syrup mixture over onion. Fold foil over onion; crimp edges to seal.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place foil packet on rack; grill, covered, 20 to 25 minutes or until onion is tender and golden. Set aside, and keep warm.
Remove fish from marinade, reserving marinade. Place marinade in a small saucepan; bring to a boil, and set aside. Place fish on rack; grill, covered, 4 to 6 minutes on each side or until fish flakes easily when tested with a fork, basting often with reserved marinade. Transfer fish to a serving platter; top evenly with onion.