Although many people are surprised by the use of ketchup in a Chinese sauce, it lends just the right amount of sweet and acidic flavors to the dish.
Coat a large 13- x 9-inch baking dish with cooking spray. Lay fillets in dish, without crowding.
Combine ketchup and next 3 ingredients in a small saucepan. Bring mixture to a boil; reduce heat, and simmer 1 minute. Remove from heat, and stir in walnuts.
Spoon sauce over halibut; bake at 450° for 12 minutes or until done. Serve with white rice and steamed asparagus.