Preheat boiler. Whisk mustard, maple syrup, lemon juice, pepper and salt as desired. Coat broiler pan with oil spray and put fillet on pan. Brush with butter, then sauce and place under broiler, Broil with oven door partially open for about 5 minutes, fish will look pink. Turn over and brush the other side of fish and continue broiling and brushing until fish flakes (not long). Serves 2.