Slice the flank steak against the grain into thin strips and season evenly with the sea salt and black pepper.
Heat the sesame oil in a large skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips in a single layer and sear until deeply browned and caramelized, about 2-3 minutes, then remove and set aside.
In the same skillet, add the broccoli florets and water, then cover with a lid for 2 minutes to steam until tender-crisp and bright green.
Remove the lid and stir in the minced garlic and grated ginger, sautéing for 30-60 seconds until the aromatics are fragrant.
Return the beef to the pan, pour in the tamari, and toss everything together for 1 minute to glaze the beef and broccoli.
Transfer the stir-fry to a plate and garnish with sesame seeds for a clean, protein-packed finish.