Slice the flank steak against the grain into thin, bite-sized strips.
In a small bowl, whisk together the tamari, grated ginger, minced garlic, red pepper flakes, and water to create the stir-fry sauce.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips to the hot pan and sear until browned and caramelized, approximately 2 to 3 minutes.
Toss in the broccoli florets and a splash of water, then cover the pan for 2 minutes to lightly steam them until tender-crisp.
Remove the lid, pour the sauce over the beef and broccoli, and stir-fry for another minute until the sauce coats everything evenly.
Season with sea salt and black pepper, then garnish with sesame seeds before serving.