In a small saucepan over medium heat, combine sake, mirin, red miso, and white sugar. Stir continuously until the mixture reaches a gentle boil and the sugar has dissolved, about 10 minutes.

Position your oven rack about 6 inches from the heat source and preheat the broiler to high. Line a baking sheet with aluminum foil.

Gently pat the black cod fillets dry with paper towels and arrange them skin side down on the baking sheet.

Pour half of the prepared miso glaze over the black cod fillets, ensuring even coating. Broil for about 3 minutes.

Carefully remove the baking sheet and pour the remaining miso glaze over the fish. Broil for an additional 8-10 minutes.

Once the top is caramelized and the fish is cooked through, serve with steamed rice and sautéed vegetables.