In a small bowl, mix together the soy sauce, rice wine vinegar, oyster sauce, sesame oil, and cornstarch. Stir until the cornstarch fully dissolves. Set the sauce aside.
Bring a large pot of water to a boil. Add the broccoli florets and cook for about 2–3 minutes, or until they turn bright green and become slightly tender. Be careful not to overcook. Drain the broccoli and rinse with cold water to stop the cooking process.
Heat 1 tablespoon of avocado oil in a wok or a large skillet over high heat. Add the beef, season with salt and pepper, and spread it out in a single layer. Sear for 1–2 minutes per side, or until the beef browns. Remove the beef from the pan and set it aside.
In the same pan, add the remaining 1 tablespoon of avocado oil. Add the minced garlic, ginger, and red pepper flakes. Stir-fry for about 30 seconds or until fragrant.
Add the blanched broccoli back into the pan and stir-fry for 2–3 minutes, or until the broccoli is tender-crisp.
Return the seared beef to the pan with the vegetables. Pour the sauce over the beef and broccoli. Stir-fry for an additional 1–2 minutes, or until the sauce thickens and coats the ingredients evenly.
Serve over steamed rice or noodles. Garnish with sesame seeds or sliced green onions, if desired.