Prepare the beef by slicing the flank steak against the grain into very thin, bite-sized strips.
In a small bowl, whisk together the tamari, minced garlic, grated ginger, and red pepper flakes to create the stir-fry sauce.
Place a large skillet or wok over medium-high heat and add the avocado oil.
Season the beef strips with sea salt and black pepper, then add them to the hot skillet in a single layer.
Sear the beef for 2 minutes until browned and caramelized, then remove the beef from the pan and set aside.
Add the broccoli florets to the same skillet with a splash of water, covering with a lid for 2 minutes to steam until vibrant green and tender-crisp.
Return the beef to the pan, pour the tamari sauce over the mixture, and toss everything together for 1 minute until the sauce thickens slightly.
Turn off the heat, drizzle with sesame oil, and garnish with toasted sesame seeds before serving.