Step 1: In a small saucepan, combine the white miso, sake, mirin, and sugar over low heat. Whisk constantly until the sugar is dissolved and the mixture is smooth. Set aside to cool completely.
Step 2: Pat the black cod fillets dry with paper towels. Place them in a non-reactive glass container and pour the cooled miso marinade over them, coating thoroughly.
Step 3: Cover the container and refrigerate for 24 to 72 hours. The longer it marinates, the deeper the flavor of this easy miso black cod recipe.
Step 4: Preheat your oven to 400°F (200°C). Gently wipe off excess marinade from the fish using your fingers.
Step 5: Heat an oven-proof skillet with oil over medium-high heat. Sear the fillets flesh-side down for 2 minutes until browned. Flip carefully.
Step 6: Transfer the skillet to the oven and roast for 8-10 minutes until the fish flakes easily and is opaque throughout.