Slice the flank steak against the grain into thin strips and season with sea salt and black pepper.
In a small bowl, whisk together the coconut aminos, rice vinegar, toasted sesame oil, 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 to the pan in a single layer and sear until browned, about 2 minutes per side, then remove and set aside.
Add the broccoli florets to the same pan with a splash of water, covering for 2 minutes to steam until tender-crisp.
Stir in the minced garlic and grated ginger, sautéing for 30 seconds until fragrant.
Return the beef to the pan and pour the sauce over the mixture, tossing constantly until the sauce thickens and coats everything beautifully.
Garnish with sesame seeds before serving hot.