Slice the flank steak into very thin strips against the grain to ensure the meat remains tender after cooking.
In a small bowl, whisk together the tamari, water, arrowroot powder, and toasted sesame oil to create the stir-fry sauce.
Heat the avocado oil in a large skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips to the hot pan in a single layer and sear for 2 minutes until browned, then remove the beef and set aside.
Add the broccoli florets to the same pan with a tiny splash of water, covering with a lid for 2 minutes to steam-fry until bright green.
Remove the lid and stir in the minced ginger, minced garlic, and red pepper flakes, sautéing for 30 seconds until fragrant.
Return the beef to the pan and pour the prepared sauce over the ingredients, tossing constantly for 1 minute until the glaze thickens and coats everything evenly.
Transfer to a plate and garnish with sesame seeds before serving.