Slice the flank steak against the grain into thin, bite-sized strips.
In a small bowl, whisk together the coconut aminos, rice vinegar, and grated fresh ginger to create the stir-fry sauce.
Heat the avocado oil in a large skillet or wok over medium-high heat until shimmering.
Add the steak strips to the pan in a single layer, seasoning with sea salt and black pepper, and sear for 2 minutes until browned.
Remove the beef from the pan and set aside, then add the broccoli florets and sliced red bell pepper to the same pan.
Sauté the vegetables for 3-4 minutes until they are tender-crisp, adding the minced garlic during the last minute of cooking.
Return the beef to the pan, pour the sauce over the mixture, and drizzle with toasted sesame oil.
Toss everything together for 1 minute until the sauce thickens slightly and coats the ingredients.
Serve immediately, garnished with toasted sesame seeds for a final crunch.