Marinate the Beef
In a bowl, combine soy sauce, cornstarch, and sesame oil. Add the sliced beef and mix well. Let marinate for at least 15 minutes.
Prepare the Sauce
In a small bowl, whisk together oyster sauce, soy sauce, sugar, beef broth (or water), and cornstarch (if using). Set aside.
Cook the Beef
Heat a wok or skillet over high heat and add 1 tablespoon of vegetable oil. Stir-fry the beef in batches for 2–3 minutes until browned and just cooked. Remove and set aside.
Stir-Fry the Aromatics
Add the remaining oil to the wok. Sauté garlic, ginger, and white parts of the green onions for 30 seconds until fragrant.
Combine Everything
Return the beef to the wok. Pour in the sauce and stir to coat evenly. Cook for another 1–2 minutes until the sauce thickens slightly. Add the green onion tops for garnish.
Serve
Serve hot with steamed rice or noodles, and enjoy!