
Place the steak strips in a bowl and add the egg. Mix together to thoroughly coat the steak in the egg.
Add the cornflour, salt, black pepper, and white pepper to the steak. Toss together to coat the steak. It will be a sticky mixture.
Heat 3 tbsp of the sunflower oil in a large frying pan or wok over high heat until very hot. Add half the beef, a strip at a time, and spread it out. Fry until dark brown and crispy, about 5-6 minutes, stirring occasionally.
Remove the beef from the pan using a slotted spoon and place it in a bowl lined with kitchen roll to soak up excess fat. Repeat with the remaining beef.
Add the remaining ½ tbsp of oil to the pan and turn the heat down to medium. Add the sliced onion and cook for 2 minutes until slightly softened.
Add the red chilli, minced ginger, and minced garlic to the pan and cook while stirring for 30 seconds.
Add the rice vinegar, dark soy sauce, tomato puree, caster sugar, tomato ketchup, and sweet chilli sauce to the pan. Turn up the heat and let it bubble for a couple of minutes until the sauce starts to reduce.
Add the beef back into the pan, give it a stir, and heat through for 1-2 minutes until the beef is hot. Serve with rice or noodles.