Slice the flank steak against the grain into thin, bite-sized strips.
Mince the garlic clove and finely grate the fresh ginger.
Heat the avocado oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips to the hot pan in a single layer, seasoning with sea salt and black pepper.
Sear the beef until browned and caramelized on the edges, about 3 minutes, then remove from the pan and set aside.
Add the broccoli florets and a tablespoon of water to the same pan; cover with a lid to steam for 2 minutes until tender-crisp.
Remove the lid and return the beef to the pan along with the garlic and ginger, sautéing for 30 seconds.
Pour in the tamari, rice vinegar, and toasted sesame oil, tossing everything together for 1 minute until the sauce coats the ingredients.
Serve immediately, garnished with sesame seeds.