Combine 1/4 cup maple syrup, teriyaki sauce, lemon juice and garlic in a large zipper bag. Add fish. Marinate in refrigerator for 20 minutes.
Prehet grill. While fish marinates, coat one side of 12-in square of tin foil. Place onions on half of tin foil. Combine 2 T maple syrup and butter, stirring well. Drizzle syrup mixture over onions. Fold foil over onion and crimp edges to seal.
Place foil packet on grill. Cover and grill for 20 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 and bring to a boil. Set aside.
Place fish on grill rack and grill 4-6 minutes on each side or until fishflakes easily with a fork, basting often with reserved marinade.