In a mixing bowl, combine the beef strips with the egg and ensure they are well coated. Gradually add in the cornflour, salt, black pepper, and white pepper, mixing until evenly covered.

Heat 3 tablespoons of sunflower oil in a frying pan or wok over high heat. Fry the coated beef strips in batches for about 5-6 minutes until dark brown and crispy. Drain on kitchen paper.

In the same pan, add the remaining oil and sauté the onion for 2 minutes. Add the chili, ginger, and garlic, sautéing for 30 seconds.

Pour in the rice vinegar, dark soy sauce, tomato puree, caster sugar, tomato ketchup, and sweet chili sauce. Stir until bubbling for 2-3 minutes until the sauce thickens.

Reintroduce the crispy beef into the pan, tossing with the sauce for another 1-2 minutes until heated through and well-glazed.