In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch, stirring until smooth. Add the thinly sliced beef, ensuring each piece is coated well. Allow the beef to marinate at room temperature for at least 15 minutes.

While the beef is marinating, wash and slice the bell peppers into thin strips and slice the onion into half-rings. Mince the garlic and grate the ginger.

Heat half of the vegetable oil in a large skillet over high heat until shimmering. Carefully add the marinated beef slices, searing for 1-2 minutes until browned on one side. Remove and set aside.

In the same skillet, add the remaining vegetable oil and sauté the sliced bell peppers and onion for 3-4 minutes. Add minced garlic and grated ginger, cooking for an additional minute.

Return the cooked beef to the skillet, pour in beef broth or water, and stir. Cook for an additional 2-3 minutes, and season with salt and pepper to taste.