Step 1: In a small saucepan, bring the sake and mirin to a boil over high heat. Boil for 20 seconds to evaporate the alcohol.
Step 2: Reduce heat to low and whisk in the white miso paste until smooth. Add the sugar and whisk until completely dissolved. Remove from heat and let the marinade cool to room temperature.
Step 3: Pat the black cod fillets dry with paper towels. Place them in a glass container or a heavy-duty Ziploc bag and pour the cooled miso marinade over them, ensuring they are fully coated.
Step 4: Cover and refrigerate for at least 24 hours, but ideally up to 72 hours for the best easy miso black cod nobu flavor.
Step 5: Preheat your oven to 400°F (200°C). Lightly wipe excess marinade off the fillets with your fingers.
Step 6: Heat oil in an oven-proof skillet over medium-high heat. Sear the fish skin-side up for 2-3 minutes until charred. Flip the fish and move the skillet to the oven.
Step 7: Roast the easy miso black cod nobu in the oven for 5-10 minutes until the fish flakes easily. Serve immediately.