Combine the first 6 ingredients in a large zip-top plastic bag. Add salmon to bag; seal. Marinate in refrigerator 40 minutes, turning once.
Remove fish from bag, and reserve marinade. Place fish on grill rack coated with cooking spray, skin side down; grill 5 minutes. Brush fish with reserved marinade; discard remaining marinade. Lightly coat fish with cooking spray. Turn fish over; grill 3 minutes or until fish flakes easily when tested with a fork. Garnish with onions, if desired.