Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, whisk together the tamari, toasted sesame oil, and arrowroot starch to create the stir-fry sauce.
Heat the avocado oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips in a single layer and sear for 2 minutes until browned, then remove from the pan and set aside.
Add the broccoli florets to the same pan with a splash of water, covering for 2 minutes to steam-fry until bright green and tender-crisp.
Stir in the minced garlic, grated fresh ginger, and red pepper flakes, sautéing for 30 seconds until fragrant.
Return the beef to the pan and pour the sauce over the mixture, tossing constantly for 1 minute until the glaze thickens and coats everything.
Garnish with sesame seeds before serving.