Slice the flank steak against the grain into very thin strips and season with sea salt and black pepper.
In a small bowl, whisk together the tamari, arrowroot powder, minced garlic, and grated fresh ginger 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 1-2 minutes per side until browned, then remove beef from the pan and set aside.
Add the broccoli florets to the same pan with a tablespoon of water, cover with a lid, and steam for 2 minutes until bright green and tender-crisp.
Return the beef to the pan, pour the sauce over the mixture, and toss constantly for 1 minute until the sauce thickens and coats everything evenly.
Remove from heat, drizzle with toasted sesame oil, and garnish with sesame seeds before serving.