In a bowl, combine sliced flank steak with soy sauce, oyster sauce, and cornstarch. Mix well and let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat.
Add the marinated steak and stir-fry for about 3-4 minutes until browned. Remove from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add garlic, onions, and bell peppers. Stir-fry for 4-5 minutes until the vegetables are tender but still crisp.
Return the beef to the skillet, pour in beef broth, and stir well. Cook for an additional 2-3 minutes until heated through.
Season with salt and pepper to taste. Serve hot over steamed rice or noodles.
Notes
For a spicier version, add sliced jalapeños or crushed red pepper flakes.
Ensure the beef is sliced thinly for a tender texture.
Serve with a side of steamed rice or noodles to soak up the sauce.