Slice the flank steak against the grain into thin, bite-sized strips.
In a small bowl, whisk together the tamari, arrowroot powder, minced 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 pan in a single layer and sear until browned, about 2 minutes per side, then remove and set aside.
Add the broccoli florets to the same pan with a splash of water, covering for 2 minutes to steam-cook until tender-crisp.
Return the beef to the pan and pour the sauce over the mixture, tossing constantly for 1 minute until the sauce thickens and glazes the ingredients.
Garnish with sesame seeds before serving.