Thinly slice the sirloin steak into strips and place in a bowl. Crack the small egg over the beef, coat evenly, then sprinkle in cornflour, salt, black pepper, and white pepper. Mix thoroughly.

Heat 3 tablespoons of sunflower oil in a large frying pan over medium-high heat. Add the beef strips in batches and fry for 5-6 minutes until dark brown and crispy. Drain excess oil on paper.

In the same pan, reduce the heat and add the remaining tablespoon of oil. Sauté chopped onion for 2 minutes, then add red chili, ginger, and garlic, cooking for an additional 30 seconds.

Pour in rice vinegar, dark soy sauce, and tomato puree. Sprinkle caster sugar, ketchup, and sweet chili sauce, stirring together. Let mixture bubble for 2-3 minutes until sauce thickens.

Return the crispy beef to the pan, tossing to coat. Heat through for 1-2 minutes and serve warm with rice or noodles.