Prepare the beef: Slice the steak very thinly against the grain.
Mix the cornflour, plain flour, salt, and five-spice powder together. Add the beef and mix until every piece is well coated.
Heat a little oil in a large frying pan. Fry the beef in batches until dark golden and crispy. Do not overcrowd the pan.
Mix all the sauce ingredients together in a bowl.
Pour off most of the oil from the pan, leaving about 1 tablespoon. Add the peppers and onion and stir-fry for 2–3 minutes, so they soften slightly but remain crisp.
Add the garlic, ginger, and chilli and cook for 30 seconds. Pour in the sauce and let it bubble for 1–2 minutes, until glossy and sticky.
Add the crispy beef and toss everything together briefly before serving, so the meat stays crispy. Serve with sticky rice and scatter the spring onions and sesame seeds over the top.