Step 1: In a small saucepan over low heat, combine the sake and mirin. Bring to a boil for 20 seconds to evaporate the alcohol.
Step 2: Reduce the heat to low and add the miso paste and sugar. Whisk constantly until the sugar has dissolved and the mixture is smooth. Let the marinade cool completely.
Step 3: Pat the black cod fillets dry with paper towels. Place them in a non-reactive container and pour the cooled miso marinade over them, ensuring all sides are coated.
Step 4: Cover the container and refrigerate for at least 24 hours, preferably 48-72 hours, for the best easy miso black cod recipe results.
Step 5: When ready to cook, preheat your oven broiler. Remove the fish from the refrigerator and gently wipe away excess marinade with your fingers or a paper towel.
Step 6: Place the fillets on a baking sheet lined with foil or parchment paper. Broil for 10-12 minutes, watching closely to ensure the top caramelizes and chars slightly without burning the whole fillet.
Step 7: Remove from the oven when the fish flakes easily with a fork. Serve immediately with your favorite sides.