Gather the ingredients. Set an oven rack about 6 inches from the heat source and preheat the oven's broiler. Line a baking sheet with aluminum foil; lightly coat with 1 teaspoon butter.

Mix together 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 cod fillets 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.