Mix mayonnaise, mustard, horseradish, lemon juice and 3 Tbsp. of the Parmesan cheese in medium bowl until well blended. Stir in 3/4 cup of the cracker crumbs.
Place fish fillets in greased or parchment-lined full sheet pan; spread evenly with mayonnaise mixture.
Mix remaining Parmesan cheese and cracker crumbs; sprinkle over coated fillets.
Bake at 350°F for 10 to 15 minutes or until fish flakes easily with fork. Serve immediately.