The popularity of cold soups in this country began with vichyssoise, originally called creme vichyssoise glacee, the chilled leek and potato mixture introduced to New Yorkers by the Frenchman Louis Diat, who cooked at the old Ritz-Carlton just after the turn of the century. Although vichyssoise is considered French, Diat actually invented it here, basing it on the classic hot leek, potato and milk soup his mother had often made, potage bonne femme (named for the French housewife). Diat has written that his mother served hot soup three meals a day, even at breakfast, and to cool off bonne femme during the warm weather, she often poured in additional milk. With that in mind, Diat added cream to the strained, pureed chicken broth containing the leeks and potatoes, and served it iced with freshly snipped chives. The "vichy" in the name comes from the famous French spa, which is located near Diat's family home.
Applying the basic method employed in vichyssoise to other vegetable purees, cooks have developed many more creamy soups over the years: soups made with corn, with green and yellow squash, with carrots, peas, lettuces, tomatoes, peppers. Because the taste of a soup loses its intensity when cold, the cook has the choice of serving the soup at room temperature or adding a strong spice -- such as curry powder or saffron -- to heighten the taste. Or a fragrance might come from herbs that complement the vegetables without overpowering them, such as fresh peas with mint or cilantro with corn.
Puree the soup in a blender or food processor and return it to the saucepan. Reheat the soup gently, and taste it for seasoning. Ladle the hot soup into bowls, garnish each one with a spoonful of creme fraiche or yogurt and a sprinkling of red onion, and serve at once. Serves four.