Mix soy sauce, sake, and mirin (or Chinese sake if you don’t have mirin) in a bowl to make the marinade. The katsuobushi (dried bonito flakes) are added to bring out the umami of the tuna, giving it a faint fishy flavor. If you don’t have any of these, a sprinkle of dashi powder can be used instead, says Prinz.
Gently place the sliced tuna into the marinade, making sure all pieces are evenly coated and soaked, and chill in the refrigerator for 30 minutes. The marinating process is very important as it not only enhances the flavor of the fish but also gives it a pleasant smoothness. If you’re in a hurry, you can marinate for a little less time, but keep in mind that this may dilute the flavor and make the tuna less smooth.