Put all the above ingredients (except the fish) in a food processor and mix until thick but smooth. (If the softness is insufficient, thin it with more sake or water.) You want a creamy texture that will adhere to the protein.

Lightly salt the fish and leave for about 20 minutes. (In the case of fish with a strong smell, salt heavily, leave to sweat, then rinse with water.) If you want to salt lightly, just sprinkle the fish, let sit, then thoroughly wipe off the fish and released juices with paper towels.

Cover the fish on all sides with plenty of the koji paste. If you want a lighter, more even finish like in a restaurant, lightly wrap the fish with gauze first and then cover it with the marinade.

Cover the coated fish with foil or place in a covered container and let it rest in the refrigerator for about 2 days.

Lightly wipe off the marinade (peel off the gauze if using gauze) and bake the fish on parchment paper in the oven at 425º for 10 minutes, or cook it under a broiler or grill it using a lightly oiled fish grill basket. Because it is easy to burn, start checking after 5 minutes.

*Yellowtail is also recommended, especially the shoulder (kama).