Fry the finely chopped onion on a medium heat with the butter, fry for about 5 minutes or when they are turning a golden colour and softening. Meanwhile, open the corned beef tin. I don't get on well with corned beef tins, my key always snaps off!! Chop the corned beef roughly and crumble into the dish. When the onion is ready pour over the top of the corned beef.