This creamy slow-cooker leek soup showcases leeks’ mild onion flavor, enhanced by plenty of fresh and dried herbs and with potatoes adding texture and body. Boursin is a soft, spreadable cheese, similar to whipped cream cheese, that comes in a variety of flavors. We like how the garlic-and-herb flavor enhances the overall flavor of the soup, but you can substitute sour cream. To prevent the sour cream from curdling in the hot soup, place in a small bowl and slowly stir in some of the hot broth until the mixture is smooth and creamy-looking before adding to the slow cooker.