Vichyssoise is a French-style soup made of puréed leeks, onions, potatoes, cream and chicken stock. Traditionally, it is served cold. 

In a saucepan over medium high heat, sauté leeks and onion in butter until tender but not brown. Add potatoes and chicken broth; bring to a boil, cover, reduce heat and simmer for 25 minutes. Puree the mixture in a food processor; return to the saucepan. Add milk and cream mixing well, add salt and pepper to taste. Refrigerate the soup until cold.

Serve vichyssoise cold and garnish with chopped chives.

It is believed that the soup was created by a French chef Louis Diat (1885-1957), who was the chef at the Ritz-Carlton in New York City for most of the first half of the 20th century.  Diat wrote about his recollection of the invention in New Yorker magazine in 1950.