Slice the thin-cut sirloin steak into strips, place them in a bowl. Add an egg, cornflour, salt, black pepper, and white pepper, mixing well until each piece is coated. Let the beef marinate for about 15 minutes.

Heat sunflower oil in a frying pan over high heat until shimmering. Carefully add marinated beef strips in batches, frying for 5–6 minutes until golden brown and crispy. Transfer them to a plate lined with kitchen paper to drain excess oil.

In the same pan, reduce heat to medium, add more oil if necessary, then sauté the sliced onion for about 2 minutes until it starts to soften. Add chopped red chili, minced ginger, and garlic, cooking for another 30 seconds until fragrant.

Pour in rice vinegar, dark soy sauce, tomato puree, caster sugar, ketchup, and sweet chili sauce. Stir well and bring to a gentle simmer, allowing sauce to cook for 2–3 minutes until it reduces slightly and thickens.

Toss the fried beef strips into the sauce, making sure they are well coated. Cook for an additional 1-2 minutes, allowing the beef to heat through and absorb the sauce flavors. Serve immediately.