Coat Grill Rack With Nonstick Cooking Spray Before Starting The Grill. In A Bowl, Combine The First Five Ingredients. Place Onions Cut Side Down On Grill Rack With Sides Touching. Arrange Fillets On Onion Slices. Cover And Grill Over Medium-Hot Heat For 5 Minutes. Baste With Mustard Mixture. Cook 5-6 Minutes Longer Or Until Fish Flakes Easily With A Fork, Basting Frequently. Discard Onion Slices.