Marinate the Beef: In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch until smooth. Add the thinly sliced beef and ensure each piece is well-coated with the marinade. Let the beef marinate for at least 15 minutes.

Sear the Beef: Heat 2 tablespoons of vegetable oil in a large skillet or wok over high heat. Carefully add the marinated beef, searing for 1-2 minutes on each side. Remove from skillet and keep warm.

Sauté Vegetables: In the same skillet, add more vegetable oil if necessary and toss in sliced bell peppers and onions. Sauté for 3-4 minutes until tender yet crisp.

Add Aromatics: Reduce heat slightly, then stir in minced garlic and grated ginger. Cook for about 1 minute until fragrant.

Combine and Simmer: Return seared beef to the skillet. Pour in beef broth or water, stirring to combine. Simmer for 2-3 minutes, thicken sauce, season with salt and pepper before serving.