Marinade: Mix sake (1 part), mirin (1 part), light colored or "usukuchi" soy sauce 薄口醤油 (1 part) and regualr or "koikuchi" soy sauce 濃口醤油 (2 parts) with dry roasted coarsely ground white sesame seeds (2 tbs) and the juice from grated ginger (1 tbs). Although white sesame seeds you get from a Japanese grocery store are already roasted, I roast them again in a dry frying pan for 5 minutes or until the sesame seeds become fragrant and just start popping. I coarsely grind them in a "suribachi" すり鉢 or a Japanese mortar. This step is really worth the effort. You should smell a nice fragrance of sesame.
Marination: In a flat sealable container, I put the marinade and the tuna slices in one layer so that all the surface of the tuna is covered with the marinade. I let it marinate at room temperature for 10 minutes and then for 2 hours in the refrigerator. The timing is rather important since the taste and texture (especially the texture) will change depending on the length of the marination. I found two hours to be perfect.
I serve this like sashimi with a Chiffonade of perilla leaves, chopped chives and thin slices of the white part of scallion (or whatever fresh herbs you like to use) with a dab of real wasabi. I think the timing of the marination, addition of sesame seeds, and ginger juice all work. Mixing light (colored) and regular soy sauce made the color of tuna sashimi darken to just the right color and make this low-grade sashimi taste and look like much better quality tuna sashimi. It tasted very good with a scant smear of wasabi. Only problem I had was that it tasted a bit too salty for me and I may reduce the proportion of soy sauce next time (my wife thought it was just fine). You could make a donburi どんぶり by placing the marinated tuna sashimi on (sushi) rice. This is indeed a great way to upgrade a low-quality tuna sashimi.