I would cut the beef as thin as you can. Heat a frying pan with just a little bit of oil. Peel an onion, leave it whole and cut down into thin rings. Break the onions into separate rings and add the onions to the pan, keep the heat low and gently fry the onion on one side. Turn the onions, add the beef and put a lid on the pan (important, it keeps everything hot and stops it going dry) and continue to fry on a gentle heat, turn everything over and give it another few minutes. Make sure everything is hot and cooked, but not burnt.