Slice the flank steak against the grain into thin, bite-sized strips.
In a small bowl, whisk together the tamari, grated ginger, minced garlic, and red pepper flakes 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 pan in a single layer, seasoning with sea salt and black pepper.
Sear the beef for 2-3 minutes until browned and caramelized, then remove from the pan and set aside.
Add the broccoli florets and 2 tablespoons of water to the same pan, covering with a lid for 2 minutes to steam until tender-crisp.
Return the beef to the pan and pour the tamari sauce mixture over the ingredients.
Toss everything together for 1 minute until the sauce thickens slightly and coats the beef and broccoli evenly.
Garnish with sesame seeds and serve immediately.