In a large resealable bag, combine the apple juice, maple syrup, soy sauce, onion and garlic. Remove 1/2 cup of the mixture and set aside in a separate bowl for basting. Place the salmon fillets in the bag and seal. Refrigerate, turning occasionally for 3 hours to marinate.
Preheat a grill for high heat. Lightly oil the grate.
Place fish fillets on the grill and discard the bag with the marinade. Grill for 5 to 10 minutes per side depending on thickness, basting occasionally with the reserved marinade.
