Flemish leek soup is one of Belgium's national dishes and a favorite during the cold and rainy winter months. To make this thick, nutritious and heartwarming dish, leeks are cooked with potatoes in a flavorful beef broth. Preisoep is most often prepared as a mere soup, but it can also be elevated to quite a meal when meaty beef bones are added to the pot, cooked until tender and reserved for the second course.