Slice the flank steak against the grain into very thin strips and season with the sea salt and black pepper.
In a small ramekin, whisk together the tamari, beef broth, and arrowroot powder until smooth to create the 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 browned, approximately 2-3 minutes, then remove the beef and set aside.
Add the broccoli florets to the same pan with a tablespoon of water; cover with a lid for 2 minutes to lightly steam until 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 beef and any accumulated juices to the pan and pour the tamari sauce mixture over the top.
Toss everything continuously for 1 minute until the sauce thickens into a glossy glaze that coats the beef and broccoli.
Transfer to a plate and garnish with sesame seeds before serving hot.