Heat the butter in a wide-bottomed pan. Sauté the leek and potatoes for approx. 3 mins. Pour in the stock and bring to the boil. Reduce the heat, simmer for approx. 20 mins. Pour in the cream, simmer for a further 10 mins., then puree and season.
Heat the oil in a non-stick frying pan. Fry the leek for approx. 5 mins. until crispy, stirring occasionally, season. Spoon the topping onto the soup.