Slice the flank steak against the grain into very thin strips to ensure maximum tenderness.
In a small bowl, whisk together the tamari, rice vinegar, and freshly grated ginger.
Heat the avocado oil in a large wok or cast-iron skillet over high heat until shimmering.
Add the beef strips in a single layer and sear without moving for 90 seconds, then toss and cook for another minute until browned; remove beef from the pan and set aside.
Add the toasted sesame oil to the same pan, then stir-fry the broccoli, bell peppers, and snap peas for 3-4 minutes until they are bright green and crisp-tender.
Add the minced garlic to the vegetables and sauté for 30 seconds until fragrant.
Return the beef and any accumulated juices to the pan and pour the tamari mixture over the top.
Toss everything together for 1 minute until the sauce has thickened slightly and evenly coated the ingredients.
Remove from heat and garnish with sesame seeds and black pepper before serving immediately.