Mix vinegar, sherry, salad dressing mix and oils until well blended. Stir in green onion.
Reserve 1/4 cup dressing mixture. Pour remaining dressing mixture over fish in shallow dish; cover. Refrigerate 30 to 60 minutes to marinate. Drain; discard dressing mixture.
Place fish on greased grill over medium-hot coals or on greased rack of broiler pan. Grill or broil 3 to 5 inches from heat 6 to 7 minutes on each side or until fish flakes easily with fork, brushing occasionally with reserved dressing mixture