I baste them in a combination of soy, sesame seed oil, a pinch of fish sauce, and ginger. I then coat them in sesame seeds and a lil pepper to give it almost a crust. I prefer to cook it on a hit skillet with just A lil oil. You drop it on their for just a few minutes on each side. You want it rare. After it cooks, cut it with the grain of the meat and let it cool. I usually serve it over baby spinach with a wasabi or ginger vinaigrette.
If it's cold (the tuna has been refrigerated), you probably want to set the tuna on the counter for a half hour before cooking. Quick searing on high heat on each side should be good. You want a very pink center as described above.