Heat the butter and oil in a large saucepan. Add the sliced and cleaned leek and cook over a low heat for 5 minutes, or until slightly softened.
Add the leftover baked potatoes, breaking them down as you stir them. Pour over the stock and bring to a gentle simmer for 10 minutes. Taste and season with salt, if required.
Serve as is for a chunky soup, or blend until smooth.
Serve in warm bowls with a swirl of cream and some freshly ground black pepper.