Firstly chop the leeks, onions and potatoes. The leeks and onions can either be in strips or cubes, up to you. The potatoes chop into medium sized pieces.
Heat the butter in a large saucepan and gently cook the leeks and onions for about 7 minutes (stir occasionally until softened and slightly golden)
Add the chopped potatoes to the pan and cook for 2-3 minutes, stirring occasionally, then add the chicken or vegetable stock and bring to the boil. Cover the pan with a tight fitting lid and simmer gently for 30-35 minutes, or until vegetables are very tender.
Season to taste with salt and pepper. Remove from the heat.