Marinate the Beef : In a bowl, combine the sliced beef with 1 tablespoon of soy sauce, the oyster sauce, cornstarch, and sugar. Mix well and let marinate for 10-15 minutes to enhance flavor and tenderness.

Prepare the Vegetables : While the beef marinates, slice the onion and mince the garlic. Set aside.

Cook the Beef : Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for 3-4 minutes until browned and nearly cooked through.

Sauté the Onion and Garlic : In the same skillet, add a little more oil if needed and sauté the minced garlic until fragrant, about 1 minute. Add the sliced onion and stir-fry for 2-3 minutes until softened but still slightly crisp.

Combine and Finish : Return the beef to the skillet with the onions. Add the remaining tablespoon of soy sauce and stir well to combine all the flavors. Season with salt and pepper to taste, adjusting to your preference.

Garnish and Serve : Transfer the beef and onions to a serving plate, garnish with sliced green onions if desired, and serve hot with steamed rice or noodles.