Marinate the Beef: In a bowl, mix thinly sliced beef with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch. Let it marinate for at least 15 minutes to enhance the flavor and tenderize the meat.
Prepare the Sauce: In a small bowl, whisk together oyster sauce, the remaining soy sauce, hoisin sauce, sweetener, and 1/2 cup of water or beef broth. Set this sauce mixture aside.
Cook the Beef: Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the marinated beef and stir-fry for about 2-3 minutes until it is just cooked through. Once done, remove the beef from the skillet and set it aside.
Cook the Onions: In the same skillet, add another tablespoon of oil. Add the sliced onions and stir-fry them for 3-4 minutes until they are soft and slightly caramelized. Stir in the minced garlic and cook for an additional minute.
Combine & Serve: Return the cooked beef to the skillet, pour in the sauce mixture, and stir-fry everything together until well coated and the sauce thickens (approximately 2-3 minutes). Garnish with chopped green onions and serve hot with steamed rice or noodles.