In a bowl, place the thin-cut sirloin steak strips and add a small egg, cornflour, salt, black pepper, and white pepper. Mix well until the beef is thoroughly coated, creating a sticky batter. Set aside for about 10 minutes.

Heat 3 tablespoons of sunflower oil in a large frying pan or wok over high heat until shimmering—about 2 minutes. Add the beef strips in batches, frying for 5-6 minutes until dark brown and crispy. Transfer cooked beef to a plate lined with kitchen paper.

In the same pan, pour in the remaining 1 ½ tablespoons of sunflower oil and reduce heat to medium. Add the sliced onion and sauté for 2 minutes. Toss in the sliced red chili, minced ginger, and chopped garlic, stirring for about 30 seconds until fragrant.

Pour in the rice vinegar, dark soy sauce, tomato puree, caster sugar, tomato ketchup, and sweet chili sauce. Stir to combine and let it simmer for 2-3 minutes until the sugar dissolves and the sauce thickens.

Return the crispy beef strips to the pan, tossing to coat the beef in the sauce. Heat through for 1-2 minutes and serve over steamed rice or stir-fried noodles.