In a bowl, mix steak strips with the egg until coated. Sprinkle cornflour, salt, black pepper, and white pepper over beef and toss until fully coated.
Heat 3 tablespoons of sunflower oil in a large frying pan or wok over high heat. Fry beef in batches for 5–6 minutes until golden brown. Remove and drain on paper towels.
Reduce heat to medium and add remaining ½ tablespoon oil. Sauté onion for 2 minutes, then add red chilli, garlic, and ginger. Stir for 30 seconds.
Pour in rice vinegar, soy sauce, tomato puree, caster sugar, ketchup, and sweet chilli sauce. Stir and bring to a boil, simmering for 2–3 minutes until thickened.
Add crispy beef back to the pan, stirring to coat in the sauce. Cook for another 1–2 minutes until heated through.
Serve immediately over rice or noodles.
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Notes
Store leftovers in an airtight container in the fridge for up to 2 days. Reheat in a hot pan to restore some crispiness.