Place the steak in the freezer for 30 minutes to make slicing easier. Then cut into thin ¼-inch strips.
In a bowl, beat the egg and mix it with the sliced beef. Add cornflour, salt, black pepper, and white pepper. Toss until the beef is well coated.
Heat 3 tablespoons of sunflower oil in a large wok or frying pan over high heat. Fry half the beef for 5–6 minutes until dark brown and crispy. Do not stir constantly—let it crisp. Remove and drain on paper towels.
Repeat with the second batch of beef using 1 tablespoon of oil.
Add the remaining ½ tablespoon of oil to the pan. Lower heat to medium and sauté the onion for 2 minutes.
Stir in the sliced chili, minced garlic, and ginger. Cook for 30 seconds until fragrant.
Add rice vinegar, dark soy sauce, tomato puree, caster sugar, ketchup, and sweet chili sauce. Stir and simmer until the sauce thickens slightly, about 1–2 minutes.
Return the crispy beef to the pan and toss to coat evenly in the sauce. Heat through for 1–2 minutes.
Serve immediately with rice or noodles.