- Place fish in large plastic food storage bag.
- Combine teriyaki marinade & sauce, mustard, brown sugar and onion powder; reserve 1/4 cup for basting.
- Pour remaining teriyaki-mustard mixture over fish. Press air out of bag; close top securely. Turn bag over several times to coat fish well. Marinate 30 minutes, turning bag over once.
- Remove fish, discarding marinade.
- Cook fish on grill, 8 to 10 inches from hot coals, 3 to 4 minutes on each side, or until fish flakes easily with fork, brushing occasionally with reserved 1/4 cup teriyaki-mustard mixture. (Or, broil fish on rack of broiler pan, 3 to 4 inches from heat source, 3 to 4 minutes on each side, or until fish flakes easily with fork, brushing occasionally with reserved 1/4 cup teriyaki-mustard mixture.)