Okay, that's weird. I'm sure you're right -- you're a professional, after all -- but what I meant by the loin is what you're calling the high twitch muscle. In Japan, it's generally labeled by a term that would normally be translated "loin," so I guess I just assumed it would be equivalent.
Koukouvagia, I was talking about the thing you're buying, that funny triangular piece that doesn't cost a mint.Sashimi
You will find that if you try to turn this "high twitch muscle" into straight-up sashimi, it doesn't block well for what you're probably used to, which is approximately rectangular slices. Now sashimi of that kind should really not be all that thin -- it's often served much too thin in American sushi joints. You should be able to get a good "bite" on it, really feel the texture between your teeth. But there are other ways when using a cut that doesn't block out this way.
If you are deft with a knife, and have a long, sharp knife, slice it with
the grain, starting down near the thin end, at an angle sort of like slicing London broil. You want your slices to be as thin as possible without unduly stretching or shredding the fish. Now stack three or four slices with about a 50% offset, by which I mean you put one sheet down, put the next on top so its middle is at one edge of the first sheet, and repeat with the next slice in the same direction. Now roll the whole thing up from the first slice to the last, trying to put your finger just a little more on the thicker, wider end of the slices. When you turn this little roll on its side, the end you had your finger on down, you can gently brush the top edges outward and you'll get a little flower rather like a rose. This one is snapper (tai
), but it works with most reddish fish:
If you know how to make sushi rice, you can do a different flower. Gently form a smallish sphere of rice, about the same size as you'd use for making nigiri-zushi, but round. Gently press down in the middle of one side to make a dimple, holding the other side flat against the stiffest part of your fingers to make it a little bit flat. Now lay your thin fish slices so the thinnest point is in the dimple and the widest part is out past the edge, and lay them gently overlapping one another, all the way around. In the middle, drop a very, very small ball of wasabi, and serve: this is supposed to look like a camellia blossom, apparently, though I wouldn't know a camellia if it bit me. Unfortunately, I can't seem to find a decent photo of this: the photos I see seem to show a single slice wrapped around the dimpled rice, which is terrific but you're not going to be able to get that kind of slice out of this "high twitch muscle" so don't kill yourself.
Last but not least, remember that if you're cutting your tuna for sashimi or camellia sushi or whatever, you're going to end up with scraps. Chop these very coarsely and toss with fresh herbs, a little olive oil, capers, perhaps a bit of very finely minced shallot or red onion, perhaps a little bit of sesame seeds, whatever... and a pinch of salt and pepper. Use some kind of round mold -- or make one with a band of folded-up aluminum foil -- and pack it into a little cake on each plate. Serve with a lemon wedge on the side. For herb choice, use whatever (not rosemary!), but be very sparing with tarragon; my preference would be minced chives and chervil, and maybe a little bit of flat parsley if you like its flavor (which you will notice here, as one often does not).