In a medium-sized bowl, mix together soy sauce, oyster sauce, and cornstarch until smooth. Add the thinly sliced beef, ensuring each piece is evenly coated in the marinade. Let sit for at least 15 minutes.

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

Heat a large skillet or wok over high heat and add vegetable oil. Once the oil is shimmering, add the marinated beef in a single layer, cooking for 1-2 minutes per side until browned. Remove beef and set aside.

In the same skillet, add more vegetable oil if needed, then toss in the sliced bell peppers and onion. Sauté for 3-4 minutes until tender but still crisp.

Add minced garlic and grated ginger to the skillet, stirring for an additional minute. Return the seared beef along with the residual marinade, pour in beef broth or water, and stir together. Cook for another 2-3 minutes.

Taste the sauce and season with salt and pepper. Once glossy and thickened, serve immediately over rice or noodles.