Combine 1/2 cup molasses, 1/4 cup mustard, and 3 tablespoons vinegar, stirring well. Place salmon and onion in a shallow dish. Pour molasses mixture over salmon and onion. Cover and marinate in refrigerator 8 hours, turning occasionally.
Remove salmon and onion from marinade, reserving marinade. Place marinade in a small saucepan. Bring to a boil; cook 1 minute. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place salmon and onion on rack; grill, covered, 4 minutes on each side, or until fish flakes easily when tested with a fork and onion is tender, basting often with reserved marinade.
Combine onion and greens. Combine remaining 1 tablespoon molasses, 1 tablespoon mustard, 2 tablespoons vinegar, oil, and salt. Pour molasses mixture over greens mixture; toss lightly. Spoon evenly onto individual serving plates. Top each serving with a fillet.