Cook soba noodles in boiling water for 4-5 minutes until tender, then drain and rinse with cold water to prevent sticking.
Thinly slice the flank steak against the grain into bite-sized strips and season evenly with sea salt and black pepper.
In a small bowl, whisk together the tamari, grated fresh ginger, minced garlic, and red pepper flakes to create the marinade.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips to the pan in a single layer and sear for 2 minutes until browned, then remove the beef and set it aside.
Add the chopped baby bok choy and sliced red bell peppers to the same pan and stir-fry for 3 minutes until the vegetables are tender-crisp.
Return the beef and cooked noodles to the pan, pour the tamari sauce over the mixture, and toss everything together for 1 minute.
Garnish the stir-fry with thinly sliced green onions and serve immediately while hot.