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 to coat and let marinate for at least 10 minutes.
02 - Whisk together 3 tbsp soy sauce, 2 tbsp oyster sauce, 1 tbsp hoisin sauce, 2 tsp cornstarch, 1/3 cup plus 1 tbsp chicken broth, 1 tsp sugar, and 1/2 tsp black pepper in a small bowl until smooth. Set aside.
03 - Heat 1 tbsp vegetable oil in a large wok or skillet over high heat. Add the marinated beef in a single layer and sear for 1-2 minutes per side until browned but not fully cooked. Remove from pan and set aside.
04 - Add the remaining 1 tbsp oil to the pan. Sauté garlic and ginger for 30 seconds until fragrant. Add broccoli florets (and onion if using) and stir-fry for 2-3 minutes until bright green and just tender.
05 - Return the beef to the pan. Pour in the prepared sauce and stir well to coat all ingredients. Continue stir-frying for 2-3 minutes until the sauce thickens and the beef is cooked through.
06 - Transfer to a serving dish and serve hot over steamed rice or noodles.