Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, whisk together the tamari, rice vinegar, and arrowroot powder to create a smooth stir-fry sauce.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat.
Add the beef strips to the skillet in a single layer and season with sea salt and black pepper.
Sear the beef until browned on both sides, then remove from the pan and set aside.
In the same skillet, add the broccoli florets with a splash of water and cover for 2 minutes to steam until bright green and tender.
Stir in the minced garlic and fresh ginger, sautéing for 30 seconds until fragrant.
Return the beef to the pan and pour the sauce over the mixture, tossing constantly until the glaze thickens and coats all ingredients.
Garnish with sesame seeds and serve immediately.