A few words about "wasabi" 山葵. Real wasabi comes from the root of the wasabi plant (Wasabi Rhizomes). If you have not ever tasted the real stuff, it may be worth it to try at least once. It tastes quite different from fake wasabis made mostly from horseradish, starch, and green dye. You can buy an American grown real wasabi root from the company called "Real Wasabi". It is expensive, does not last that long and needs a special grater (the traditional one is made of shark skin). I tried it once from this company. It was really good but, unless you are having a big sushi and sashimi party, it may be difficult to justify the purchase. Some of wasabi pastes in a tube can be quite good using a certain amount of real wasabi but you can also buy a real wasabi powder from this company.
Soy sauce 醤油 is also important. Many of my American colleagues have told me they think soy sauce will last for a long time (basically forever) but it will get oxidized fairly quickly once it is opened. Oxidized soy sauce tastes quite unpleasant (to me, at least). You could make a special soy sauce for sashimi (dried bonito flakes and mirin are needed) but you could also buy special soy sauce for sashimi in a bottle.
Tuna in grated mountain yam "Yamakake" 鮪のやまかけ
The name of this dish is-- "yama-kake", "yama" comes from the word "yama-imo" which means mountain potato and "kake" which means to pour over. So the literal translation is "poured over mountain potato". I grate the "mountain" yam or "yama-imo" 山芋 after peeling the skin. In the United States, I can only get a domesticated version of yama-imo called "naga-imo" or long potato 長芋. A Japanese style grater (one with multiple holes allowing the grated yam to drop into the lower container) or a Japanese mortar "suribachi" works best. I also dissolve wasabi paste in soy sauce and mix it into the grated potato. The more traditional way of doing this would be to put a dab of wasabi on the side of the dish letting the diners mix it in with the soy sauce. I like my method better because with the traditional way, the wasabi paste often does not get evenly distributed and the person eating it can get a real jolt of concentrated wasabi.
Add the tuna cubes to the grated mixture and garnish with a large amount of "nori" seaweed. The slimy consistency of the grated yam may not agree with many Westerners but this is certainly my wife's favorites.