01 - Combine cornflour, plain flour, salt, and pepper in a bowl. Dip beef strips into beaten egg, then coat thoroughly with flour mixture. Shake off excess coating.
02 - Heat vegetable oil in a deep pan or wok to 350°F. Fry beef in batches for 3–4 minutes until golden and crispy. Remove and drain on kitchen paper.
03 - Whisk together soy sauce, sweet chilli sauce, rice vinegar, tomato ketchup, dark brown sugar, and sesame oil in a small bowl until combined.
04 - Pour off most oil from wok, leaving about 1 tablespoon. Stir-fry garlic, onion, peppers, and red chilli over high heat for 2 minutes until just softened.
05 - Pour sauce into wok and bring to a simmer for 1 minute, allowing flavors to meld.
06 - Add crispy beef to wok and toss quickly to coat evenly with sauce. Stir in spring onions. Serve immediately, sprinkled with sesame seeds and coriander if desired.