In a small bowl, mix together the soy sauce, rice wine vinegar, oyster sauce, sesame oil, and cornstarch. Stir until the cornstarch is fully dissolved. 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 oil in a wok or a large skillet over high heat. Add the beef, season with salt and pepper, and spread them out in a single layer. Sear for 1-2 minutes per side, or until the beef is browned. 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.