How to Make It
In a large pot of boiling water, cook the soba until al dente, 6 minutes. Drain, cool under running water and shake out excess water; transfer to a large bowl.
Meanwhile, in a small bowl, beat the eggs with the sugar and a pinch of salt. In a medium nonstick skillet, heat the vegetable oil. Add the eggs and cook over moderate heat, tilting the pan to form a very thin omelet. Cook on one side only, just until golden on the bottom and the top is barely set, about 3 minutes. Cover, remove from the heat and let stand until the top is just set, about 5 minutes. Slide the omelet onto a board and cut it into very thin strips.
Add the omelet strips, carrot, cucumber, radishes, mirin and 1/4 cup of the soy sauce to the soba and toss; transfer to bowls.
Chop the tuna with the scallions; transfer to a bowl. Stir in the Sriracha, sesame oil and the remaining 2 teaspoons of soy sauce. Mound the tuna on the soba, sprinkle with the sesame seeds, top with a dab of wasabi and serve right away.