Prepare the Beef:

1. Slice the Beef: Thinly slice the beef against the grain to ensure tenderness. If you’re using flank steak, this step is crucial for achieving a melt-in-your-mouth texture.
2. Coat the Beef: In a bowl, toss the sliced beef with cornstarch until each piece is evenly coated. This will help create the crispy texture when fried.

Make the Sauce:

3. Combine the Sauce Ingredients: In a separate bowl, whisk together the soy sauce, hoisin sauce, rice vinegar, sugar, and sesame oil. Add the chili flakes if you prefer a bit of heat.
4. Prepare Cornstarch Slurry: Mix the cornstarch with water to create a slurry, which will be used to thicken the sauce later.

Cook the Beef:

5. Fry the Beef: Heat vegetable oil in a large skillet or wok over medium-high heat. Add the beef in batches and cook until it’s golden and crispy on the outside, about 2-3 minutes per batch. Remove the beef from the pan and set it aside.
6. Sauté the Aromatics: In the same skillet, add a bit more oil if necessary and sauté the garlic and ginger for about 30 seconds until fragrant.

Cook the Vegetables:

7. Stir-Fry the Vegetables: Add the bell peppers and onion to the skillet and stir-fry for about 2-3 minutes until they are tender-crisp and slightly caramelized.

Combine and Simmer:

8. Add the Sauce and Beef: Return the cooked beef to the pan with the vegetables and pour the sauce over the mixture. Stir to combine and coat the beef and vegetables in the sauce.
9. Thicken the Sauce: Add the cornstarch slurry to the pan and cook for another 1-2 minutes until the sauce has thickened and is glossy.

Garnish and Serve:

10. Garnish and Serve: Serve the Beijing Beef hot, garnished with sliced green onions and sesame seeds if desired. Pair it with steamed rice or noodles for a complete meal.