In a medium saucepan, combine equal parts sake and mirin over medium heat. Allow to boil for 1-2 minutes to evaporate alcohol.

Reduce heat and stir in white miso paste and granulated sugar until smooth and glossy. Remove from heat and let cool.

Coat black cod fillets in miso mixture and refrigerate in a dish or bag for at least 24 hours, preferably 48.

Preheat oven to 400°F (200°C) and let fillets sit at room temperature for 15 minutes before cooking.

Heat vegetable oil in a skillet over medium-high heat. Sear marinated fillets skin-side up for 2-3 minutes until golden.

Flip fillets skin-side down and transfer to preheated oven, baking for 5-8 minutes until opaque.

Allow cooked fish to rest in the skillet for a few minutes before serving. Enjoy!