Preheat oven to 350 degrees F (175 degrees C). 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 tablespoon 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 tablespoon Parmesan cheese. Sprinkle over the coated halibut.
Bake for 15 to 18 minutes, or until fish flakes easily with a fork.