Slice the flank steak against the grain into very thin strips.
Heat the avocado oil in a large wok or cast-iron skillet over high heat until it begins to shimmer.
Add the beef strips in a single layer and sear without moving for 1-2 minutes to develop a deep brown crust, then toss and cook for 1 more minute.
Remove the beef from the pan and set aside on a plate.
Add the broccoli florets and beef broth to the same pan, covering with a lid for 2 minutes to steam the broccoli until tender-crisp.
Remove the lid and stir in the minced ginger, garlic, and red pepper flakes, sautéing for 30 seconds until fragrant.
Whisk together the tamari and rice vinegar in a small bowl, then pour it into the pan along with the cooked beef.
Toss everything together for 1 minute to thicken the sauce slightly and coat the ingredients.
Turn off the heat and drizzle with toasted sesame oil.
Garnish with sliced green onions and sesame seeds before serving.