Slice 360g of thin-cut sirloin steak into strips. Whisk together 1 small egg, 4 tbsp cornflour, ¼ tsp salt, and black and white pepper in a mixing bowl. Toss beef strips in the mixture until evenly coated. Set aside.

Heat 3 tbsp sunflower oil in a large frying pan over high heat. Add beef strips in batches and fry for 5-6 minutes until dark brown and crispy, flipping halfway. Drain on paper towels.

In the same pan, add 1 tbsp oil and lower heat to medium. Add sliced onion and sauté for about 2 minutes until softened. Stir in red chili, minced ginger, and minced garlic, cooking for additional 30 seconds.

Stir in 2 tbsp rice vinegar, 3 tbsp dark soy sauce, 2 tbsp tomato puree, 6 tbsp caster sugar, 2 tbsp tomato ketchup, and 2 tbsp sweet chili sauce. Bubble for 2-3 minutes until glossy and slightly thickened.

Add the cooked crispy beef back into the pan and toss in the sauce for 1-2 minutes until well coated and heated through. Serve hot with steamed jasmine rice or stir-fried noodles.