Marinade: I made some adjustment and used a more simple and less potent sake and soy sauce mixture (3 tbs each or 1:1 ratio) with the addition of grated ginger (1/2 tsp, I used grated ginger but you could add just the juice) and dry roasted (in a dry frying pan) white sesame seeds (1/2 tsp, coarsely ground in a Japanese mortar or in "suribachi" すり鉢). Compared to the original, I do not use any mirin and the amount of soy sauce is less.
Tuna: I used blue-fin tuna (slightly fatty or ko-toro 小トロ portion) which was from Catalina Offshore Porducts. I did the usual "yubiki" 湯引き process to make the surface of the tuna white (10 seconds in simmering water) and then cooled down immediately in ice water.
I sliced the tuna in rather thick pieces as seen above and marinated it in a flat sealable container in one layer for 2-3 hours in the refrigerator.
To serve, I sliced Vidalia spring onion after halving it and placed the slices on the bottom of the plate, added a few strips of shaven carrot for color, surrounded that with sliced (on a slant) American mini cucumber. I drizzled the marinade and olive oil over them. Since I had daikon left after making a daikon garnish for sashimi the night before, I grated it and squeezed out the excess moisture, added yuzu juice and made a small mound in the center of the plate.
After removing the excess marinade from the "zuke" pieces, I arranged the slices around the grated daikon. I added a small dab of real wasabi on the side. To eat, we put a bit of wasabi on the zuke slice, added some onion, carrot and cucumber slices on top. The tuna has a remarkable resemblance to good beef. The marinade did not make the tuna "slimy" but infused lots of flavors. The real wasabi is really not too hot like the "fake" kind but has a nice fresh wasabi flavor. Sweet Vidalia is also perfect with this combination. If you feel the zuke is a bit salty (not here though) or regradless, you could add yuzu infused grated daikon on the top as well. This proves that if you make zuke from good tuna, the end result is better. For a drink, there is not much choice--it has to be cold sake.