Mix Parmesan, 1/3 cup butter, mayonnaise, green onion, lemon juice, dill, salt, black pepper, and hot sauce in a medium bowl until well combined. Set aside.
Lay fish in a single layer on the prepared baking sheet.
Cook under the preheated broiler until fish flakes easily with a fork, 7 to 8 minutes.
Gently spread Parmesan mixture over fish; return to the oven and broil until topping is bubbly and lightly browned, about 2 more minutes.