Creating Pepper Steak with Bell Peppers and Onion is straightforward if you follow these steps carefully:

1. Prepare the Steak: Slice the flank steak or sirloin thinly against the grain for tenderness.
2. Marinate: In a bowl, combine soy sauce, oyster sauce (if using), salt, and pepper. Add the sliced steak and marinate for about 10 minutes.
3. Heat Oil: In a large skillet or wok, heat olive oil over medium-high heat.
4. Cook Steak: Add the marinated steak to the skillet, cooking until browned, about 2-3 minutes. Remove the steak and set it aside.
5. Sauté Peppers and Onion: In the same skillet, add the sliced onion and bell peppers. Stir-fry for about 3-4 minutes until they are slightly tender yet crisp.
6. Add Garlic: Add minced garlic to the skillet and cook for an additional minute until fragrant.
7. Combine: Return the steak to the skillet with the peppers and onions, stirring to combine.
8. Thicken Sauce: Add the cornstarch mixture and stir until the sauce thickens, about 1-2 minutes.
9. Taste and Adjust: Check for seasoning and adjust with salt and pepper as necessary.
10. Serve: Serve the finished dish over cooked rice, allowing the flavorful sauce to soak into the grains.

These simple steps will guide you in creating a delicious and satisfying Pepper Steak with Bell Peppers and Onion.