God, this looks good, doesn't it? And it's just the leftovers! When I saw local tuna for sale at the farmers market, I remembered a technique the amazing Tadashi Ono taught me called maguro zuke, an old Tokyo style of marinating tuna, which he serves to great acclaim at Matsuri restaurant. With this dish, you infuse the outside of the tuna with classic soy sauce and mirin flavors, but the inside remains raw and tender. It's a simple method that yields incredible results -- I prepared it for a dinner party last night and my guests flipped (but not literally). Today, my wife and I enjoyed it for lunch, sliced thin over steaming rice and accented with chiffonade of shiso leaf, a la the picture above, a dish called maguro don in Japan. My wife literally flipped. (Okay, not literally.)
The secret of this dish is to quickly -- repeat, quickly -- parboil the tuna. That is, dunk the sucker in boiling water for 10 seconds so a cooked outside layer forms, while the inside remains raw and tender. Cool in an ice bath. I did this with two tuna steaks, each about an inch thick. The cooked surface now acts like a sponge, and will soak up the flavors of a marinade. I made a marinade, following what Tadashi showed me, of roughly 3 parts soy sauce to 2 parts mirin -- so a little more savory than sweet. For the soy sauce, I mixed Japanese dark soy sauce (koikuchi shoyu) with intensely flavored tamari, to give it a stronger flavor kick (I'm not sure what Tadashi would think of this, I have to ask him). I bathed the tuna in the marinade for half a day in the fridge, sliced and served. Leave any extra tuna soaking in the marinade; it will become even more flavorful the next day and the day after. Thank you Tadashi!