Slice the beef thinly against the grain. In a medium mixing bowl, combine the beef with 1 tablespoon of soy sauce, cornstarch, and a pinch of salt and pepper. Mix well and let marinate for 10–15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef in a single layer, working in batches if necessary to avoid overcrowding. Cook for 1–2 minutes per side, just until browned but not fully cooked through. Remove the beef from the pan and set aside.

In the same pan, add the remaining 1 tablespoon of vegetable oil. Toss in the sliced onions and cook for 3–4 minutes, stirring occasionally, until they soften and begin to caramelize.

Push the onions to one side of the pan and add the minced garlic to the empty space. Sauté the garlic for 30 seconds to release its aroma, then mix it with the onions.

Pour in the remaining tablespoon of soy sauce and the oyster sauce (if using). Stir everything together to coat the onions evenly in the sauce.

Return the beef to the pan along with any juices that have accumulated. Stir-fry everything together for 1–2 minutes, just until the beef is cooked through and the sauce clings to the ingredients.

Taste the dish and adjust the seasoning with additional soy sauce, salt, or pepper if needed. Garnish with sliced green onions, if desired, and serve hot.