Heat the oil in a large saucepan over a medium heat then add potatoes, onions and leeks. Cook and stir for about 5 minutes or until starting to go soft. Add stock and bring to the boil then reduce heat and simmer until tender, about 20 minutes; season to taste.
Blend the soup in a food processor or with a handheld stick blender until smooth. Return soup to a clean pan to warm through, hot water can be added here if needed to thin soup. Remove from heat and stir in creme fraiche. Ladle into warmed soup bowls and sprinkle over some fresh chopped parsley; serve.