Wash the 400 grams sablefish, scraping away any scales or blood on the surface. Pat the filets dry with paper towels.
Add the 10 grams ginger, ¼ cup sake, ¼ cup mirin, ¼ cup soy sauce, and 2 tablespoons sugar to a pan that's just big enough to fit the fish and bring the mixture to a boil.
Add the black cod to the pan and use a spoon to baste the tops of the fish with the braising liquid periodically.
After a few minutes, flip the filets over and continue basting them with the sauce.
Continue flipping and basting every few minutes until the fish is cooked through and the sauce has thickened into a thick glaze.