Preheat air fryer to 375°. Combine mayonnaise, 2 tablespoons cheese, mustard seed and pepper; spread over tops of fillets.
Place fish in a single layer in greased air-fryer basket. Cook until fish flakes easily with a fork, 3-5 minutes.
Meanwhile, in a small bowl, combine bread crumbs, onion, ground mustard and remaining 1 tablespoon cheese; stir in butter. Spoon over fillets, patting gently to adhere; spritz topping with cooking spray. Cook until golden brown, 2-3 minutes longer. If desired, sprinkle with additional green onions.