Making Crispy Beijing Beef is a straightforward process if you follow these steps:

1. Prepare Beef: Slice the flank steak against the grain into thin strips.
2. Marinate: In a bowl, combine soy sauce, cornstarch, rice vinegar, vegetable oil, sesame oil, garlic, ginger, and red pepper flakes. Add the beef strips and toss to coat. Let marinate for at least 30 minutes.
3. Heat Oil: In a large skillet or wok, heat oil over medium-high heat until shimmering.
4. Fry Beef: Remove the beef from the marinade, allowing excess liquid to drip off. Fry the beef in batches, without overcrowding the pan, until crispy and cooked through, about 3-4 minutes per batch. Remove and drain on paper towels.
5. Stir-Fry Vegetables: In the same skillet, add bell peppers and stir-fry for 2-3 minutes until tender-crisp.
6. Combine: Return the crispy beef to the skillet and toss with the bell peppers and green onions. Stir-fry for an additional minute to heat through.
7. Serve Immediately: Transfer the Crispy Beijing Beef to a serving platter.

These steps will guide you in creating an incredible dish that will impress everyone at your table.