Marinate the Beef: In a bowl, combine the beef slices with soy sauce, oyster sauce, cornstarch, and sesame oil. Mix well and let marinate for at least 15 minutes.

Prepare the Chinese Broccoli: Trim the ends of the gai lan and cut the stems and leaves into manageable pieces. If the stems are very thick, you can slice them in half lengthwise

Stir-Fry the Beef: Heat 1 tablespoon of oil in a wok or large skillet over high heat. Add the marinated beef and stir-fry until it's just cooked through. Remove the beef from the wok and set it aside.

Cook the Gai Lan: Add the remaining oil to the wok. Stir-fry the garlic until fragrant, then add the gai lan. Stir-fry for a few minutes until the stems start to soften.

Combine Ingredients: Add the oyster sauce, soy sauce, and water or beef broth to the wok. Bring to a simmer and let the gai lan cook until it's tender but still slightly crisp.

Finish the Dish: Return the beef to the wok, mixing well with the gai lan and sauce. Stir-fry for another 1-2 minutes. Adjust seasoning with salt and pepper.

Serve: Serve hot, accompanied by steamed rice.