1. In a bowl, add and mix together the white miso paste, cooking sake, mirin and sugar together until it forms a smooth paste for the marinade. Set aside.
2. Sprinkle salt over the fish and set aside for about 30min. Salt will draw excess water/liquid and smell away from the fish.
3. Gently pat dry the fish with paper towel to remove the moisture. Do not wash with water!
4. Pour half of the marinade sauce into a flat bottom container. Place the fish in the container and slather the rest of marinade sauce over the fish to coat both sides with the marinade.
5. Cover with salan wrap and let the fish marinade in the fridge for 2-3 days.
Cook the cod with oven
Preheat oven to 200 ºC. Wipe excess miso marinade off of the fillets with your hands (don't rinse it off) and place the fish skin side up on a baking pan lined with parchment paper or silicone baking sheet. Then bake about 10 to 15 minutes or until the fish is almost cooked through.