Thinly slice the flank steak against the grain into bite-sized strips to ensure maximum tenderness.
In a small glass bowl, whisk together the tamari, grated fresh ginger, minced garlic, and red pepper flakes to create the stir-fry sauce.
Heat the avocado oil in a large cast-iron skillet or wok over medium-high heat until it begins to shimmer.
Add the steak strips in a single layer, seasoning with sea salt and black pepper, and sear for 2 minutes until deeply browned and caramelized.
Remove the beef from the skillet and set aside on a plate.
Add the broccoli florets to the same pan with a splash of water, then cover with a lid for 2 minutes to steam-fry until they are bright green and crisp-tender.
Return the beef and any accumulated juices to the pan, pour in the prepared sauce and the toasted sesame oil.
Toss everything together for 1 minute until the sauce thickens slightly and coats every piece.
Garnish with thinly sliced green onions and toasted sesame seeds before serving immediately.