Slice the flank steak against the grain into thin, bite-sized strips and season evenly with sea salt and black pepper.
In a small bowl, whisk together the tamari, water, arrowroot powder, and red pepper flakes to create a smooth stir-fry sauce.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips to the hot pan in a single layer and sear until deeply browned, about 2 minutes, then remove the beef and set it aside on a plate.
In the same pan, add the broccoli florets with a small splash of water, cover with a lid, and steam for 2 minutes until bright green and tender-crisp.
Remove the lid and stir in the minced garlic and grated fresh ginger, sautéing for 30 seconds until the aromatics are fragrant.
Return the cooked beef and any accumulated juices to the pan, then pour the prepared sauce over the mixture.
Toss everything constantly for 1 minute until the sauce thickens into a glossy glaze that coats the beef and broccoli.
Remove from heat and garnish with sesame seeds before serving immediately.