In a medium bowl, combine soy sauce, oyster sauce, and cornstarch until smooth. Add the beef steak slices, ensuring each piece is evenly coated. Let the beef marinate for at least 15 minutes.

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

Heat a large skillet over high heat and drizzle in vegetable oil. Once the oil shimmers, add the marinated beef slices. Sear for 1-2 minutes on each side until browned, then remove and set aside.

In the same skillet, add more vegetable oil if needed, then add the sliced bell peppers and onions. Stir-fry for 3-4 minutes until tender-crisp.

Add minced garlic and grated ginger, stirring for 30 seconds. Return the cooked beef to the skillet, pour in beef broth or water, and bring to a simmer for 2-3 minutes until the sauce slightly thickens.

Taste and adjust the seasoning with salt and pepper. Serve hot over rice or alongside noodles.