Thinly slice the flank steak against the grain into bite-sized strips and season with sea salt and black pepper.
In a small glass bowl, whisk together the tamari, toasted sesame oil, minced ginger, minced garlic, and arrowroot powder to create a smooth stir-fry sauce.
Heat the avocado oil in a large stainless steel skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips in a single layer and sear for 2-3 minutes until browned and caramelized, then transfer the beef to a plate and set aside.
Add the broccoli florets to the hot pan with a tablespoon of water, then cover with a lid for 2 minutes to steam until they are bright green and tender-crisp.
Return the beef and any accumulated juices to the pan, pour the sauce over the mixture, and toss constantly for 1 minute until the sauce thickens and glazes the ingredients.
Remove from heat and garnish with sesame seeds before serving immediately.