Dust both sides of the sablefish fillets with sea salt. Cover the fillets in plastic wrap and transfer to the refrigerator. Let sit for 15–20 minutes.
Wash salt off the fillets with very cold water. Blot dry with a paper towel.
Tear the konbu into pieces the size of your fillets. Wet a new paper towel with sake and use it to moisten the konbu. Sandwich the sablefish between pieces of sake-moistened konbu. Cover the fillet in plastic wrap and refrigerate for 30–40 minutes. Remove the konbu and return the fillet to the refrigerator.
Mix the soy sauce, sugar, mirin, and katsuobushi with 1 1/2 tablespoons of sake, and 1 1/2 tablespoons of water, in a saucepan. Bring to a boil, then lower heat and simmer for about 10 minutes. Drain and remove the katsuobushi then set the sauce aside.
In a small bowl, combine 8 tablespoons of cold water with 2 tablespoons of potato starch to create a thickener (add the water to the potato starch gradually, whisking constantly to avoid clumping). Return the soy/mirin sauce to a boil then lower heat to a simmer. If desired, add the potato starch thickener to the sauce, gradually, until the desired consistency is reached. (Some people may choose to add very little or no thickener—you definitely won't want to use it all, but it's easier to mix a large batch.) Remove from heat and let cool.
To serve:
Slice sablefish into portions approximately 1 inch wide by 2 inches long. Lightly char one side of the fish with a small butane torch or sear it very briefly in a hot saucepan. Top fish with a drizzle of the sauce and a sprinkle of sesame seeds. Serve slices of faux-nagi over bowls of hot steamed rice. You can also serve this
nigiri style as they do at Tataki.
*Seafood Watch® recommends wild-caught sablefish
from Alaska and British Columbia.
Learn more about Seafood Watch