Mix soy sauce, oyster sauce, and cornstarch in a bowl and coat the beef slices thoroughly. Let the beef soak up the flavors for at least 15 minutes.
Heat a tablespoon of vegetable oil in a skillet or wok over high heat and quickly brown the beef slices for about 1-2 minutes per side, then set them aside.
Add the remaining oil to the skillet and toss in the bell peppers and onion, cooking them until they soften yet remain crisp, roughly 3 to 4 minutes.
Stir in the minced garlic and grated ginger, cooking briefly until you notice their aroma, about one minute.
Return the beef to the skillet, pour in the beef broth or water, blend everything and continue to cook for 2-3 minutes until the sauce begins to thicken.
Season with salt and pepper to taste and serve hot alongside rice or noodles for a hearty meal.