Chop the onions into rings. Add a splash of oil to the pan and cook the onions on a high heat for 2 or 3 minutes. Reduce the heat and put on lid. Cook until the onions start to soften. Add a splash of beer or a sprinkle of sugar to help them to caramelise. Cook until very soft, this will take at least 30 minutes, possibly longer. Dont be tempted to rush this step or you wont get the depth of flavour.