Rinse leek and cut into rings.
Rinse sorrel, shake dry and remove thick stems. Cut into strips and set aside 1/4 cup of strips for garnish.
Heat oil in a pan and sauté leeks, shallots and garlic until translucent. Deglaze with vegetable broth. Peel and finely grate the potato and add to the pot of broth. Bring broth to a boil, reduce heat, and simmer for 20 minutes. Add chopped parsley and sorrel. Puree the soup. Return the soup to a simmer. Add cream and creme fraiche. Season with salt and pepper to taste. To serve, ladle into soup bowls and sprinkle with reserved sorrel.