Creating Crispy Chilli Beef is simple if you follow these steps:

1. Marinate the Beef: In a bowl, toss the thinly sliced beef with soy sauce, rice vinegar, and a sprinkle of salt and pepper. Let it marinate for about 15 minutes.
2. Prepare the Coating: Place the cornstarch in a shallow dish. Coat each slice of marinated beef evenly in cornstarch, ensuring there’s a good layer on each piece.
3. Heat Oil: In a large frying pan or wok, heat the vegetable oil over medium-high heat until shimmering.
4. Fry the Beef: Add the coated beef slices to the hot oil in batches. Fry until golden brown and crispy, about 3-4 minutes on each side. Remove and drain on paper towels.
5. Sauté Vegetables: In the same pan, add sliced onions, bell peppers, minced garlic, and ginger. Stir-fry for 2-3 minutes until the vegetables are tender.
6. Combine Sauces: Add oyster sauce, sweet chili sauce, and red chili flakes to the vegetable mix. Stir well to combine and heat through.
7. Toss Beef: Return the crispy beef to the pan. Toss everything together, ensuring the beef is well coated with the sauce and mixed with the vegetables.
8. Garnish: Remove from heat and garnish with spring onions and sesame seeds for an added touch.
9. Serve Immediately: Crispy Chilli Beef is best served hot and fresh.

Each of these steps will guide you toward creating a dish that is as delicious as it is visually enticing.