Put the beef in a bowl and toss in the cornflour and five-spice. Heat the oil in a wok or large frying pan until hot, then add the beef and fry until golden and crisp. Scoop out the beef and drain on kitchen paper.
Pour away all but 1 tbsp oil.Add the pepper, half the chilli, the white ends of the spring onions, garlic and ginger to the pan.
Stir-fry for 3 mins to soften, but don’t let the garlic and ginger burn. Mix the vinegar, soy, chilli sauce and ketchup in a jug with 2 tbsp water, then pour over the veg.
Bubble for 2 mins, then add the beef back to the pan and toss well to coat.
Serve the beef on noodles with prawn crackers, if you like, scattered with the remaining chilli and the green parts of the spring onions.