|Cut away any deep red part of the tuna, and cut the rest of the fish into four 1-inch thick pieces. In a suribachi or other mortar, grind the sesame seeds until they are just broken.
In a pan large enough to hold the steaks in one layer, combine 1/2 cup shoyu, the mirin, and sesame seeds. Place the tuna in a pan, and marinate it for 15 minutes.
For the salad dressing: In a large bowl, combine the mustard, 1 teaspoon shoyu, the komezu, grated ginger, sugar, sesame oil, and olive oil, and whisk until smooth.
To cook the fish: In a large deep skillet, heat 1 inch vegetable oil over medium heat to 360°F.
While the oil heats, drain the tuna, discarding the marinade but reserving the sesame seeds. Coat the tuna with the sesame seeds, and then the flour.
Add the tuna to the skillet, and cook the tuna, turning it with tongs and a spatula, until both sides are golden. The center of the fish should be rare and pink, but cook it longer if you like.
Remove the fish from the oil, and drain it on a rack for 10 minutes. With a very sharp knife, cut the tuna diagonally to produce slices 14-inch thick.
Toss the salad greens with the dressing, and serve the tuna on the of greens, garnished with tomatoes.