Follow these simple steps to create your crispy and flavorful dish:

1. Prep the Beef: Start by slicing the beef thinly against the grain. This ensures tenderness.
2. Marinate: In a bowl, mix the sliced beef with soy sauce, sesame oil, salt, and pepper. Allow it to marinate for at least 10 minutes.
3. Coat the Beef: In a separate bowl, add the cornstarch. Dredge the marinated beef in cornstarch until fully coated.
4. Heat the Oil: In a large skillet or wok, heat vegetable oil over medium-high heat until it’s shimmering.
5. Cook the Beef: Carefully add the coated beef to the hot oil in batches to avoid overcrowding. Fry until golden brown and crispy, about 3-5 minutes. Remove and drain on paper towels.
6. Stir-fry the Vegetables: In the same skillet, add garlic, red chillies, onion, and bell pepper. Stir-fry for about 2-3 minutes until just tender.
7. Combine: Return the crispy beef to the skillet with the vegetables. Add sweet chilli sauce and rice vinegar. Toss everything together to coat the beef and veggies.
8. Finishing Touches: Once heated through, sprinkle with chopped green onions and sesame seeds if desired.
9. Serve Immediately: Transfer to a serving platter and enjoy hot for the best texture and flavor.

Following these clear and concise steps will guide you to achieve an amazing Crispy Chilli Beef that captures the essence of delicious Asian cuisine.