Prepare the Beef Coating: In a mixing bowl, combine the thin-cut sirloin steak strips with one small egg, four tablespoons of cornflour, a quarter teaspoon of salt, a quarter teaspoon of black pepper, and an eighth teaspoon of white pepper. Toss to ensure they are evenly coated.

Fry the Beef: Heat a frying pan over medium-high heat and add four and a half tablespoons of sunflower oil. Add small batches of coated beef strips and fry for approximately 5-6 minutes until crispy and dark brown. Transfer to a paper towel-lined plate.

Sauté the Aromatics: In the same pan, reduce heat slightly and add one medium chopped onion, sautéing for about two minutes. Then, add one sliced red chili, one teaspoon of minced ginger, and three minced garlic cloves. Stir for 30 seconds.

Make the Sauce: Pour in two tablespoons of rice vinegar, three tablespoons of dark soy sauce, two tablespoons of tomato puree, six tablespoons of caster sugar, two tablespoons of tomato ketchup, and two tablespoons of sweet chili sauce. Stir and simmer for 2-3 minutes.

Combine and Serve: Return the crispy beef to the pan with the sauce, tossing to coat. Heat for an additional 1-2 minutes and serve immediately with steamed jasmine rice or stir-fried noodles.