1. Prepare the Beef

Begin by slicing the flank steak (or your choice of beef) against the grain into thin strips, about 1/4-inch thick. This step is crucial as slicing against the grain ensures the beef remains tender and easy to chew. Once sliced, set the beef aside. If you’d like, you can marinate the beef for 10-15 minutes in a small amount of soy sauce or cornstarch for added flavor and tenderness, though this step is optional.

2. Make the Sauce

In a small bowl, whisk together the soy sauce, oyster sauce, hoisin sauce, rice vinegar, brown sugar, and black pepper. This mixture combines sweet, savory, and tangy flavors, forming the base of your sauce. In a separate small bowl, mix 1 teaspoon of cornstarch with 2 tablespoons of water to create a slurry. This will help thicken the sauce once it’s added to the stir-fry.

If you prefer a spicier kick, add 1 tablespoon of chili paste or sriracha to the sauce mixture, adjusting the heat level to your preference.

3. Stir-Fry the Vegetables

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Once the oil is hot, add the sliced bell peppers and onions. Stir-fry for about 2-3 minutes until they begin to soften but still retain a slight crunch. Add the minced garlic and grated ginger (if using) and stir-fry for another 30 seconds, allowing the garlic and ginger to become fragrant. Remove the vegetables from the pan and set them aside.

4. Cook the Beef

In the same skillet or wok, add the remaining 1 tablespoon of vegetable oil. Once the oil is hot, add the sliced beef to the pan in a single layer. Let it sear for about 1-2 minutes on each side until it is browned and slightly crispy on the edges. You may need to cook the beef in batches to avoid overcrowding the pan. This ensures that the beef gets a good sear and doesn’t steam.

Once the beef is cooked, remove it from the skillet and set it aside.

5. Combine the Beef and Vegetables

Return the cooked beef and vegetables to the skillet. Stir to combine. Pour the sauce over the beef and vegetables, stirring to coat everything evenly. Bring the mixture to a simmer, and allow it to cook for an additional 1-2 minutes. As the sauce simmers, it will begin to thicken, coating the beef and vegetables with its savory flavor.

6. Thicken the Sauce

Once the sauce has simmered for 1-2 minutes, add the cornstarch slurry (cornstarch mixed with water) to the pan. Stir constantly until the sauce reaches your desired thickness. If the sauce becomes too thick, you can add a small amount of water or broth to loosen it up.

7. Garnish and Serve

Once the sauce has thickened, remove the skillet from the heat. Garnish the dish with sliced green onions and sesame seeds (if desired) before serving. Serve immediately with steamed rice, noodles, or even over a bed of sautéed vegetables for a complete meal.