In a mixing bowl, whisk together soy sauce, oyster sauce, and cornstarch until smooth. Add the sliced beef, ensuring each piece is well coated with the marinade. Let it marinate at room temperature for at least 15 minutes to allow the flavors to penetrate the meat, while you prepare your vegetables.

While the beef marinates, slice the bell peppers and onion into thin strips, aiming for uniformity to ensure even cooking. Mince the garlic and grate the ginger, setting them aside.

Heat a large skillet or wok over high heat, adding vegetable oil to create a shimmering surface. Once the oil is hot, carefully add the marinated beef in a single layer. Sear for 1-2 minutes until nicely browned, then flip to cook the other side for an additional minute. Remove the beef and set aside on a plate.

In the same skillet, add the sliced bell peppers and onion. Stir-fry for 3-4 minutes, keeping the vegetables tender yet crisp. Once they start to soften, add the minced garlic and grated ginger, cooking for an extra minute until fragrant.

Return the cooked beef to the skillet with the sautéed vegetables. Pour in the beef broth or water, stirring to combine everything. Cook for another 2-3 minutes, allowing the sauce to thicken slightly and coat the beef and vegetables. Season with salt and pepper to taste before serving this delicious pepper steak hot over rice or noodles.