How to Make It
Preheat the oven to 350°. Spread the sesame seeds on a small rimmed baking sheet and toast until browned, tossing once, about 6 minutes. Transfer the seeds to a spice grinder and let cool completely. Process until finely ground.
In a large pot of boiling water, cook the soba noodles until just tender. Drain and cool under cold running water. Transfer the noodles to a bowl of ice water to chill. Drain well.
Meanwhile, in a large bowl, combine the toasted sesame seeds with the vinegar, lime juice, lime zest, kombu tsuyu, tamarind, brown sugar, garlic and gochugaru. While whisking constantly, slowly drizzle in the sesame oil until incorporated. Season with salt. Add the cold noodles along with the cucumber, celery, cilantro, scallions and chile. Season with salt and toss. Transfer to bowls; serve.
Kombu tsuyu is a Japanese condiment made from dashi soup stock, soy sauce, mirin and sugar. It’s available at Japanese markets and on amazon.com.