Put all of the sauce ingredients into a saucepan, bring to a boil, then leave on a simmer to thicken whilst the beef is being cooked.
In a large oven tray or a bowl, toss the beef strips through the cornflour and salt. Use your hands and give the beef a few squeezes to make sure that the cornflour is tightly packed on.
Heat the oil in a frying pan over a high heat, then add the beef and fry for a few minutes until golden and crispy. You may find it easier to do this in two batches, before returning the first batch to the pan.
By now the sauce should have reduced down to a thick and glossy state that can coat the back of a wooden spoon. If it is still quite thin, turn up the heat and stir continuously to reduce it down. Pour the sauce over the beef strip and toss in the peppers stir through to coat every bit of crispy beef.
Serve with rice or noodles and garnish with spring onion.