Combine the mayonnaise, 2 Tbsp. 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 until fish flakes easily with a fork, 4-5 minutes.
In a small bowl, combine the bread crumbs, onion, ground mustard and remaining cheese; stir in butter. Spoon over fillets; spray topping lightly with cooking spray. Broil until golden brown, 1-2 minutes longer. Sprinkle with green onions.