PreparationStep 1: Marinate the BeefIn a bowl, mix the soy sauce, oyster sauce, cornstarch, and sesame oil.Add the beef slices to the marinade and toss to coat evenly. Let sit for at least 15 minutes (up to 30 minutes for deeper flavor).Step 2: Prepare the SauceIn a small bowl, whisk together the soy sauce, hoisin sauce (if using), black pepper, and the cornstarch slurry. Set aside.Step 3: Stir-Fry the VegetablesHeat 1 tablespoon of vegetable oil in a large skillet or wok over high heat.Add the onion and bell peppers. Stir-fry for 2–3 minutes, until slightly softened but still crisp. Remove and set aside.Step 4: Cook the BeefAdd the remaining 1 tablespoon of oil to the skillet.Add the marinated beef in a single layer and sear for 1–2 minutes per side until browned but not fully cooked.Add the garlic and ginger, stirring for 30 seconds until fragrant.Step 5: Combine and FinishReturn the cooked vegetables to the skillet with the beef.Pour in the prepared sauce and toss everything together. Stir-fry for another 2–3 minutes, or until the sauce thickens and coats the beef and vegetables evenly.