Step 1: In a medium bowl, combine the thinly sliced flank steak with 2 tablespoons of soy sauce, cornstarch, and 1 tablespoon of vegetable oil. Toss well to ensure the beef is evenly coated. Let it marinate at room temperature for at least 15 minutes, or refrigerate for up to 30 minutes.
Step 2: While the beef marinates, prepare your vegetables. Slice the onion and both bell peppers into thin strips. Mince the garlic and fresh ginger (if using) and set them aside. This ensures everything is ready for quick stir-frying.
Step 3: Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat until shimmering. Add the marinated beef in a single layer, ensuring not to overcrowd the pan. Stir-fry for 2-3 minutes, until the beef is browned and just cooked through. Remove the cooked beef from the pan and set it aside on a plate.
Step 4: Add the sliced onions and bell peppers to the same hot pan. Stir-fry for 3-4 minutes, or until they begin to soften but still retain a slight crispness. Add the minced garlic and ginger, stir-frying for an additional 30 seconds until fragrant.
Step 5: Return the cooked beef to the pan with the vegetables. Pour in the 1/4 cup of soy sauce, oyster sauce (if using), and sprinkle in the black pepper. Toss everything together vigorously, ensuring the beef and vegetables are thoroughly coated in the sauce. Cook for another 2 minutes to heat through and allow the flavors to meld.
Step 6: Remove the pan from the heat and drizzle with sesame oil for an aromatic finish. Serve the Sizzling Chinese Pepper Steak immediately over freshly cooked rice or noodles. For an authentic presentation, consider serving on a preheated sizzling plate!