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.
⅓ cup apple juice, ⅓ cup maple syrup, 3 tablespoons soy sauce, 2 tablespoons finely chopped onion, 4 (8 ounce) salmon fillets, 2 cloves garlic, minced