Place the flank steak in the freezer for 15 minutes to firm up, then slice against the grain into paper-thin strips.
In a medium bowl, whisk together the tamari, toasted sesame oil, grated fresh ginger, minced garlic, sea salt, and black pepper.
Add the beef strips to the marinade, tossing well to coat, and let marinate at room temperature for 10 minutes.
Heat a large non-stick skillet or wok over high heat until very hot.
Add the beef to the skillet in a single layer and sear for 1-2 minutes per side until deeply browned and caramelized.
Toss in the broccoli florets and sliced red bell pepper, stir-frying with the beef for an additional 3 minutes until the vegetables are crisp-tender.
Divide the warm cooked brown rice into a bowl and top with the beef and vegetable mixture.
Garnish with thinly sliced green onions and toasted sesame seeds before serving.