Heat the oil in a wok or large frying over a high heat.
Pat the beef dry with some kitchen roll, then mix the cornflour (cornstarch) 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. Try not to move the meat around too much as this reduces the amount of crispiness you get. It should take about 5-6 minutes to crisp up the beef, with about 3 or 4 stirs during that time.
Once the beef is crispy and cooked through, remove the beef using a slotted spoon, and place it in a bowl lined with kitchen roll to soak up excess fat.
After the beef has been removed from the pan there should still be a little oil left. Turn the heat down to medium and add the sliced red pepper and onion.
Fry for 2-3 minutes (so they're hot but still crunchy).
While the peppers and onions are cooking, mix the orange zest, juice, soy sauce, sugar, garlic, rice vinegar, and ginger and put to one side.
Pour over the orange sauce. Turn up the heat and bring to the boil. Simmer for 1 minute.
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 cornflour slurry to thicken.
Once bubbling and thick, serve up with boiled rice. Garnish with the spring onions (scallions).
