This is a refreshing rice bowl with marinated fresh tuna (maguro) and grated nagaimo yam. The key to the flavor is the cooked soy sauce mixture. I usually sprinkle shiso leaves on top, but you can use chopped green onion or grated wasabi instead. You can also lightly sear the surface of the tuna block.
Put the soy sauce, sake and mirin in a pan and bring to the boil to evaporate the alcohol. Let cool.
Slice the tuna block to 5 mm thick. Put into the boiled soy sauce combination, cover with plastic wrap and marinate in the refrigerator for about an hour.
Put the rice into a bowl and let it cool down a bit. Grate the nagaimo yam in a mortar and put 2/3 on top of the rice.
Arrange the tuna slices on top of the grated nagaimo in the bowl, and pour the rest of the nagaimo on top. Drizzle on some of the marinade to taste, and put the garnishes on top.
Story Behind this Recipe
I was making a cooked soy sauce mixture with a combination of ingredients that I liked, and decided to try marinating some tuna in it.
The cooked soy sauce mixture is mild and more rounded than if you used regular soy sauce or sashimi soy sauce. If you marinate the tuna for too long, the meat will harden, so pay attention. If you have to leave it in the marinade for a long time, wrap the tuna block in paper towels beforehand. When serving, put the grated nagaimo on top of the hot rice first so that the heat from the rice won't cook the tuna.