In a small bowl, mix soy sauce, rice vinegar, oyster sauce, sesame oil, and cornstarch until smooth.
Bring water to a boil and blanch broccoli for 2–3 minutes until bright green. Drain and rinse with cold water.
Heat 1 tablespoon avocado oil in a wok over high heat. Add beef, season with salt and pepper, and sear for 1–2 minutes per side. Remove and set aside.
Add remaining oil to the pan. Stir-fry garlic, ginger, and red pepper flakes for 30 seconds.
Add broccoli and stir-fry for 2–3 minutes.
Return beef to the pan and pour in sauce. Cook 1–2 minutes until thickened.
Serve your Beef and Broccoli Stir Fry over rice or noodles.