Heat the oil in a large pan. Toss the chopped beef in the cornflour, salt and pepper. Fry the meat in the oil for about 5 mins on medium to high heat. You can do this in batches, but I'm a bit too lazy for that. I find that if you put the meat in, and leave it alone for a few minutes before you stir it, you'll get a nice dark colour on the beef. The beef will probably stick a bit to the pan, so you might need to give it a little scrape with a spatula.