Heat the oil in a wok or large frying. Pat the beef dry with some kitchen roll, then mix the cornflour with the salt and pepper and toss the beef in the cornflour until completely coated. When the oil is hot, tip the beef in and fry until crispy. I try not to move the meat around too much as this seems to reduce the amount of crispiness you get. It generally takes about 9 or 10 minutes to crisp up the beef, with about 3 or 4 stirs during that time. cook the beef in batches, I always use a very hot oil. Once the beef is crispy and cooked through, remove the beef using a slotted spoon, and place in a bowl lined with kitchen roll to soak up excess fat.
Whilst the beef is cooking, mix the orange juice, zest, sugar, soy sauce, garlic, ginger and rice vinegar and put to one side.
After the beef has been removed from the pan there should still be a little oil left. Turn the heat down a little and add the sliced red pepper and onion. Fry for a minute or two (so they're hot but still crunchy), then pour over the orange sauce. Turn up the heat and bring to the boil, then add the beef back in. Give it a stir and heat through for 1 minute. If your sauce is looking too thin, you can stir in a little bit of cornflour mixed with cold water to thicken.
Once bubbling and thick, serve up with boiled rice. Garnish with the spring onions/scallions.