Chilled Arugula-Potato Soup with Buttermilk

This smooth soup has definite arugula character, thanks to a two-step procedure: First, leaves are simmered with leeks and potatoes to make a mellow, thick base; then raw leaves pureed with buttermilk are added for bright color and a light mustardy nip. Serve on a summer’s eve to precede a meal of cold meat, cucumbers, tomatoes, and dark bread.