In a mixing bowl, whisk together soy sauce, oyster sauce, and cornstarch until smooth. Add thinly sliced beef to the bowl, tossing to ensure each piece is well-coated. Let the beef marinate for at least 15 minutes at room temperature.

Heat vegetable oil in a large skillet or wok over high heat until shimmering. Carefully add the marinated beef in a single layer and sear for 1-2 minutes on each side until browned. Remove and set aside.

In the same skillet, pour in the remaining vegetable oil. Once hot, add sliced bell peppers and onion, stirring frequently. Sauté for 3-4 minutes until tender yet crisp.

Introduce minced garlic and grated ginger to the skillet, stirring continuously for 1 minute until fragrant.

Return the seared beef to the skillet, along with any remaining marinade. Pour in beef broth or water, stirring everything together. Cook for an additional 2-3 minutes until the sauce thickens slightly.

Taste and adjust the seasoning with salt and pepper before serving hot.