Cock-a-leekie soup is a Scottishsoup dish of leeks and chicken stock, often thickened with rice, or sometimes barley. The original recipe added prunes during cooking, and traditionalists still garnish with a julienne of prunes.  Anne Mulhern of Glasgow's Willow Tearooms suggests that the reason for the addition of prunes dates back to times when only boiling fowls were available and prunes were added to increase the nutritional value of the broth.
While it is called “Scotland’s National Soup,” it probably originated as a chicken and onion soup in France. By the 16th century, it had made its way to Scotland, where the onions were replaced with leeks. The first recipe was printed in 1598, though the name “cock-a-leekie” did not come into use until the 18th century.