Heat the butter in a large saucepan until bubbling and frothy (or use coconut oil until melted). Fry the leeks in it for 5 mins on a low-medium heat until soft, then add the potatoes and stock (start with 500ml/2 cups) and bring to the boil. Simmer for 15-20 mins, until the vegetables are soft and cooked. Blitz until smooth in a food processor or with a hand blender. Add more stock if you prefer a thinner soup. Season and add herbs, if using. Serve hot. Add a splash of cream or milk (dairy or unsweetened non-dairy) if you like. Great served with homemade bread and cheese or hummus.