Heat the oil in a medium saucepan, add the leek and potato and fry for 2–3 minutes until softened.
Add the vegetables to the pan with the mint and the stock and bring to the boil. Reduce the heat and simmer for 10 minutes.
Transfer the soup to a food processor or blender and process until smooth. Return to the pan with the crème fraîche and season with salt (if liked) and pepper. Heat through and serve.