Combine the mayonnaise, 2 tablespoons cheese, mustard seed and pepper; spread over tops of fillets. Place on a broiler pan coated with cooking spray. Broil 4 in. from the heat for 3-5 minutes or until fish flakes easily with a fork.
Meanwhile, in a small bowl, combine the bread crumbs, onion, ground mustard and remaining cheese; stir in butter. Spoon over fillets; spritz topping with cooking spray. Broil 1-2 minutes longer or until golden brown. Yield: 4 servings.