Special note from August 4, 2010: First, while the original post follows, I thought it time for a 2010 update, which you can read here. Second, the photo above is one I took. I've noticed some other blogs using it. That's fine. If you do, much obliged if you could credit Kitchen Monkey. Thanks! Now read.
This is one of the best appetizers ever, and also happens to be one of the easiest to make. One of the joys of Sarasota (apart from it being January 12 with a high of 80 degrees) is the bounty of seafood and fishmongers. In fact, I happen to live less than a block away from a restaraunt/seafood-market that sells sushi-grade tuna and yellowtail, my two favorites. The guy behind the counter is very professional, and he won't let you take the fish until he has wrapped it in paper, then enclosed it in a plastic bag filled with ice to keep it superfresh--even when you explain to him that you live across the street.
And yes, to make tataki you really need your tuna to be sushi-grade. I must warn you though: if you are lucky enough to have easy access to it, there is a danger of it becoming habit-forming. The drawback to having such easy access to such great tuna is that I'm not a rich man, and the stuff can run $17 a pound or more, so I have instead had to learn a degree of self control. Sometimes as I pass the market, I shake my fist in the air at the injustice of it all.
I find that for four people, about 2/3 pound is ideal, so you're looking at anywhere from $9 to $12. Worth it if you ask me. Ask the monger to try and cut it off the tail end; if he cuts it off the large end, you're bound to get a steak that is too thin for good tataki. You ideally want a chunk that, if not square, is about 1 1/2 inches high and 2 1/2 inches wide.
Once you've got it home, sprinkle sesame seeds liberally onto all four sides, this gives it a little bit of crunch without changing the flavor significantly. At medium-high, heat 1 parts veggie oil to 1/2 part pure sesame oil in a pan, and with a pair of tongs sear the tuna on four sides. Don't sear it for very long! Just enough so that about an eighth of an inch cooks through on each side. If you like you can sear the two ends as well. Immediately remove from the pan, and slice thinly, no more than a quarter of an inch slices.
Serve as soon as possible. I find it works best with this simple variation of ponzu sauce: mix 2 parts soy sauce with 1 part lemon juice and 1 part orange juice. Garnish with shredded daikon or just eat.