Slice the flank steak against the grain into thin strips and season evenly with sea salt and black pepper.
In a small mixing bowl, whisk together the coconut aminos, rice vinegar, grated fresh ginger, and arrowroot powder 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 until browned, about 2 minutes per side, then transfer the beef to a plate 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 tender-crisp.
Remove the lid, add the minced garlic, and sauté for 30 seconds until fragrant.
Return the beef and any accumulated juices to the pan, then pour the prepared sauce over the mixture.
Toss everything together for 1 minute until the sauce thickens and coats the beef and broccoli.
Remove from heat, drizzle with toasted sesame oil, and garnish with sesame seeds before serving.