This chilled velouté is made with humble ingredients, but with a little technique, the leeks, potato, and silky crème fraîche come together for a bowl of pure luxury. Though its namesake is the spa town of Vichy, France, the cold version of this potato soup was invented at the Ritz-Carlton Hotel in New York City.
Melt butter in a stockpot over medium heat. In a second stockpot over low heat, warm the chicken stock. Meanwhile, slice the white parts of the leeks in half lengthwise; then thinly slice. Trim and thinly slice celery. Add leeks and celery to the melted butter; gently sweat until translucent, 7–8 minutes, adding a pinch of salt halfway through. Make sure the vegetables do not take on any color.Meanwhile, slice potatoes lengthwise into 4 wedges, then roughly dice into 1-inch chunks. When the leeks and celery are translucent, add warmed stock, followed by the potatoes.
Make a sachet: Lay a square of cheesecloth large enough to fit the herbs on a flat surface. Place thyme, parsley, bay leaf, and garlic in the middle; then wrap into a bundle and tie with butcher’s twine. Submerge the sachet in the soup and simmer, uncovered, until potatoes are soft, 15–20 minutes.
When a fork easily pierces a potato, the soup is ready to be blended. First, remove the sachet. Then use a ladle to remove 1½ cups of broth; set aside. Carefully add the rest of the soup to a blender and cover. Blend, beginning on low speed, then slowly increasing to high speed. When the soup is pale and completely puréed, turn off the blender and add crème fraîche. Cover and continue blending on high speed to a light, smooth texture. Test the thickness, then add some of the reserved broth as necessary to thin the soup, and blend again. Set aside the remaining broth.
Strain the soup (optional): In a large bowl filled with ice, chill a second bowl large enough to hold the soup. Strain the soup through a chinois or fine-mesh strainer into the chilled bowl; use the back of a ladle to help push the soup through the strainer. The vichyssoise will have a syrupy texture when it’s warm, but it will become thicker and more velvety as it chills in the bowl. Let cool for 1 hour.
When the vichyssoise is chilled, strain more reserved broth into the soup as needed to thin to desired consistency; stir well to combine. Ladle into a bowl. Shape quenelles of crème fraîche (optional): Pass about a tablespoon of crème fraîche back and forth between two spoons, smoothing and shaping it until you have formed an egg-shaped dollop, or "quenelle." Garnish each bowl with a quenelle of crème fraîche and several chive batons before serving.