Toss the beef with the cornflour, salt and pepper until well coated.
Heat the vegetable oil in a wok or large frying pan over high heat.
Add the beef and fry for 2-3 minutes until crispy and golden brown.
Remove the beef from the pan and set aside.
Add the garlic and chilli to the pan and fry for 1 minute.
Add the soy sauce, rice vinegar, brown sugar and sesame oil and stir to combine.
Return the beef to the pan and toss to coat in the sauce.
Serve immediately, garnished with the sliced spring onions.
