2 lb. fresh-frozen, thawed white fish fillets (1 1/2 inch thick)
1/2 tsp. salt
1/8 tsp. ground pepper
6 tbsp. (3/4 stick) butter
1 med. onion
3 c. fresh bread crumbs
1 c. grated cheddar cheese
1/4 c. chopped fresh parsley
1 tsp. dry mustard

Preheat oven to 350 degrees. Lightly grease 9 x 12 inch baking dish. Season fillets with salt and pepper. Arrange fillets in baking dish.

Melt butter in large skillet over medium-low heat. Add onion and stir until tender, about 5 minutes. Add remaining ingredients and mix well. Spread topping over fillets. Bake until fish is opaque and flakes easily and topping is golden brown (about 20-25 minutes). Serves immediately.