Preheat oven to 350°F. Lightly grease a baking sheet.

In a small bowl, mix together mayonnaise, mustard, horseradish, and lemon juice. Stir in 1/4 cup bread crumbs and 1 tbsp Parmesan cheese. Arrange fish fillets on the prepared baking sheet. Spread bread crumb mixture evenly over fish.

In a small bowl,mix together melted margarine,1/4 cup bread crumbs, and 1 tbsp. Parmesan cheese. Sprinkle over the coated halibut.

Bake for 15 - 18 minutes, or until fish flakes easily with a fork.