Prepare the Beef: Cut the beef into thin strips, about 1/4 inch thick. The thinner the strips, the crispier they will become. Slicing against the grain will ensure the beef is tender. Use a sharp knife for clean cuts.

Marinate the Beef: In a bowl, combine all the marinade ingredients with the beef strips. Mix well, ensuring all the beef is coated in the marinade. Marinate for at least 15 minutes, or longer if you have time. The longer the beef marinates, the more flavorful it will become. You can even marinate the beef overnight in the refrigerator for maximum flavor.

Prepare the Stir Fry Sauce: In a separate small bowl, mix all the stir fry sauce ingredients and set aside. Having the sauce ready to go before you start cooking makes the stir-fry process much smoother and faster. Whisk the sauce ingredients together until the sugar and salt are dissolved.

Coat the Beef: Break the egg into the marinated beef strips and mix well. This helps the cornstarch adhere to the beef. Then, coat the beef strips with the corn/potato starch. Make sure all the beef strips are separated and evenly coated. Toss off any excess flour before frying. This prevents the oil from becoming too starchy and helps the beef to crisp up nicely. Shake off any excess cornstarch from the beef before frying to prevent clumping.

Fry the Beef: Heat the oil in a wok or large pan to high heat (about 350°F or 175°C). Use a deep-fry thermometer to check the temperature if you have one. If you don’t have a thermometer, the oil is ready when a small piece of bread dropped into it turns golden brown in about 30 seconds. Carefully add the coated beef strips to the hot oil and deep fry for 1-2 minutes, until crispy and golden brown. Don’t overcrowd the pan; fry in batches if necessary. Overcrowding the pan will lower the oil temperature and prevent the beef from crisping up properly. Remove from the oil with a slotted spoon and transfer to a paper towel-lined plate or cooling rack to drain excess oil. Fry the beef in small batches to maintain the oil temperature and ensure even cooking.

Stir-Fry the Vegetables: Heat a tablespoon of oil in the wok/pan over medium-high heat. Add the cut onions and stir-fry for a few seconds until slightly softened. Then add the bell pepper slices and stir-fry for another minute or two until they are tender-crisp. Don’t overcook the vegetables; you want them to retain some of their crunch. Stir-fry the vegetables quickly over high heat to preserve their texture and color.

Combine and Serve: Add the fried beef strips to the wok/pan with the vegetables. Pour the stir fry sauce over the beef and vegetables. Add the spring onions and fresh chillies. Stir well to combine all the ingredients and cook over high heat for 1-2 minutes, until the sauce has thickened slightly and everything is heated through. The high heat and quick cooking time ensure the beef remains crispy. Toss the beef and vegetables in the sauce until everything is evenly coated and the sauce has thickened.

Serve Immediately: Transfer the Crispy Chilli Beef to a serving plate and serve immediately. This dish is best enjoyed hot and crispy. The longer it sits, the softer the beef will become.