1Bring a large pot of water to a rolling boil. Add soba noodles and cook according to package directions until just al dente, about 4-5 minutes. Drain immediately and rinse under cold running water until completely cooled. Shake off excess water thoroughly and set aside.
2Combine soy sauce, mirin, and Worcestershire sauce in a small bowl, whisking together until fully blended. Set this sauce mixture within arm's reach of your cooking station.
3Heat a large wok or heavy-bottomed skillet over high heat until smoking hot, about 2-3 minutes. Add 1½ tablespoons vegetable oil and swirl to coat the pan.
4Toss in chicken pieces and sear without moving for 1-2 minutes until golden on one side. Flip and cook until just cooked through, about 2-3 minutes more. Remove to a plate and set aside.
5Add remaining 1½ tablespoons vegetable oil to the same wok. Toss in minced garlic and ginger, stirring constantly until fragrant but not browned, about 20 seconds.
6Add sliced onion to the wok and stir-fry over high heat until softened and starting to caramelize at the edges, about 2 minutes.
7Throw in bell peppers and mushrooms, tossing continuously with a quick flicking motion for 2-3 minutes until vegetables are tender-crisp with slight char marks.
8Add the cooled soba noodles to the wok, using tongs or chopsticks to separate any clumps. Toss vigorously for 1 minute to heat through.
9Pour half the prepared sauce over the noodles and toss rapidly to coat every strand, about 30 seconds. Add remaining sauce and the cooked chicken, continuing to toss until everything is evenly coated and the sauce has reduced slightly, about 1-2 minutes.
10Add beansprouts and green onions, tossing just until the beansprouts are barely wilted but still crunchy, about 30 seconds. Remove from heat immediately.
11Drizzle sesame oil over the noodles and give one final toss to distribute.
12Transfer to serving plates or bowls, mounding the noodles attractively. Sprinkle generously with bonito flakes and dust with aonori seaweed powder for authentic Wagamama presentation.