Slice the potatoes about ½cm thick. Heat 3cm olive oil in a large frying pan and add the onions. Cook gently for about 10 minutes until they start to soften, then add the potatoes and a couple of really good pinches of salt to the pan and keep cooking on a low heat, turning carefully now and again for another 20-30 minutes until the potatoes are completely tender. Try not to get too much colour on anything – you want them to stew in the oil rather than crisp up. Don’t worry if they start to break up a little.