Aritsugu Tsukiji is in Tsukiji market, Tokyo. Aritsugu Kyoto is in Nishiki market, Kyoto. They're not the same company. The Tsukiji stuff is good and oddly cheap. The Kyoto stuff is good--but nobody would call it cheap.
In the US, it is unfortunately true almost always. It is not necessary, however, except with freshwater fish, including salmon, and a very few others. This is why salmon sashimi is quite rare in Japan.
I would urge you not to ferment/age this way. If you're going to do that, you must control it with salt. This is very normal with some fish, but not tuna. The kombu wrap method should help a lot, and you can probably get away with a full day of refrigerated aging, but no more.
If you like that aged flavor, may I suggest katsuo-tataki? I am told you can get whole katsuo loins (or whole fish) in Fulton market if you ask around. Then you just rinse well, salt lightly, wait...
I am no professional, but here are some points.
1. The Japanese pros say, "with fish, wash it once, wash it twice." Rinse the block of tuna very well under cold, running water, then blot dry with paper towels.
2. Most fish in serious restaurants are not simply sliced raw, but rather seasoned carefully. With maguro, this really shouldn't be necessary, but you season based on your actual ingredients, not on theory. If you are getting noticeable improvement from slight...
Good God. I'dve thought this was long dead.
Japanese draw-cutting is almost invariably done with the index finger on the spine. One effect, paradoxically, is that you put essentially no force into the cut, and let the weight of the knife and its motion do the work. This is ideal for a lot of fish work. With vegetable cutting, the question doesn't really arise, because an usuba is held in so many grips depending on the cuts and materials.
If you learned that one should...
More or less piggyback on everyone else: you can and should cook the soup part for hours to marry the flavors, but the proteins -- most especially seafood -- go in at the last minute and poach gently.
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