Cook the soba noodles in boiling water according to package directions, then drain and set aside.
Slice the flank steak into very thin strips against the grain and season with sea salt and black pepper.
Heat the avocado oil in a large wok or skillet over medium-high heat until shimmering.
Add the beef strips to the wok in a single layer and sear until browned and caramelized, about 2 minutes, then remove and set aside.
Add the broccoli florets and sliced red bell pepper to the same pan with a tablespoon of water, covering for 2 minutes to steam-cook until tender-crisp.
Stir in the minced garlic and grated ginger, sautéing for 30 seconds until the aroma is released.
Return the cooked beef and the drained soba noodles to the wok.
In a small bowl, whisk together the tamari, toasted sesame oil, and rice vinegar, then pour the mixture over the noodles.
Toss everything together over high heat for 1 minute until the sauce is well distributed and the dish is piping hot.