Slice the flank steak against the grain into very thin strips and season with sea salt and black pepper.
In a small bowl, whisk together the tamari, toasted sesame oil, minced garlic, grated ginger, and red pepper flakes.
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 for 1-2 minutes until browned, then remove beef from the pan and set aside.
Add the broccoli florets to the same pan with a splash of water, cover with a lid, and steam for 2 minutes until bright green and tender-crisp.
Return the beef to the pan, pour the sauce over the mixture, and toss everything together for 1 minute until the sauce thickens slightly and coats the ingredients.
Plate the stir-fry immediately and garnish with sesame seeds for a final nutty finish.