01 - Combine the sliced beef with 2 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 1 tsp rice vinegar in a bowl. Mix thoroughly and let marinate for at least 10 minutes.
02 - Whisk together all sauce ingredients in a small bowl: soy sauce, oyster sauce, hoisin sauce, cornstarch, chicken broth, sugar, and black pepper. Set aside until ready to use.
03 - Heat 1 tbsp vegetable oil in a large wok or skillet over high heat. Add the marinated beef in a single layer. Sear for 1-2 minutes per side until browned but not fully cooked through. Remove beef from pan and set aside.
04 - Add remaining 1 tbsp oil to the pan. Add minced garlic and ginger, sautéing for 30 seconds until fragrant and aromatic.
05 - Add broccoli florets (and sliced onion if using). Stir-fry for 2-3 minutes until broccoli turns bright green and becomes just tender-crisp.
06 - Return the seared beef to the pan. Pour in the prepared sauce, stirring constantly to coat all ingredients. Continue stir-frying for 2-3 minutes until sauce thickens and beef is cooked through. Serve hot over steamed rice or noodles.