In a mixing bowl, combine the thinly cut sirloin steak with the beaten small egg, ensuring each strip is coated evenly. Gradually add the cornflour, salt, black pepper, and white pepper, mixing until the beef is well-covered.

Heat 3 tablespoons of sunflower oil in a wok over high heat until shimmering. Fry the coated beef strips in small batches for about 5-6 minutes or until dark brown and crispy, turning occasionally for even cooking. Drain on kitchen paper.

In the same wok, add the remaining 1.5 tablespoons of sunflower oil and heat over medium heat. Sauté the sliced onion for 2 minutes until softened, then add red chili, minced ginger, and minced garlic; sauté for 30 seconds until fragrant.

Stir in rice vinegar, dark soy sauce, tomato puree, caster sugar, ketchup, and sweet chili sauce into the wok. Cook for 2-3 minutes, allowing it to bubble and slightly reduce.

Return the cooked crispy beef to the wok, tossing gently in the sauce for 1-2 minutes until heated through and well-coated.