Thinly slice the flank steak against the grain into bite-sized strips to ensure maximum tenderness.
In a small bowl, whisk together the tamari, grated fresh ginger, minced garlic, and toasted sesame oil 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 without moving for 2 minutes to develop a deep brown crust.
Toss the beef and add the broccoli florets along with a tablespoon of water, then cover with a lid for 2 minutes to lightly steam the vegetables.
Remove the lid, pour the sauce over the beef and broccoli, and stir-fry for another 1-2 minutes until the sauce thickens and becomes glossy.
Season with sea salt, black pepper, and red pepper flakes, then garnish with sesame seeds before serving.