This is Chef Traci Des Jardins' version of the classic French soup. Here she updates the standard recipe with the addition of crème fraîche and sorrel. The result is a rich, silken soup that's delicious hot or cold, depending on the season.
Melt the butter in a heavy-bottomed saucepan and add the onions and leeks. Sauté over low heat, making sure not to color the vegetables. When the onions and leeks are translucent and soft, add the potatoes and some salt, and cover with the stock. Cook at a simmer for about 30 to 45 minutes or until the potatoes are completely soft.
Transfer to a blender and purée until smooth. If desired, pass the soup through a fine sieve set over a bowl. Add the crème fraîche and season to taste with salt and white pepper. Chill if serving the soup cold, or ladle into bowls if serving hot.
Cut the sorrel into a fine dice and quickly sauté in the olive oil until just wilted. Garnish the soup with the sorrel and chives.