Chilled Watercress, Spring Nettle, And Sorrel Soup

In a large saucepan, warm the olive oil and butter over medium heat until the butter is foamy. Add the onion, leeks, garlic, and salt, and stir to coat. Let cook 10 minutes, then add 4 cups stock or water. Bring to a simmer. Add the watercress and nettles and simmer just until tender. Using a fine-mesh sieve over a bowl, strain the soup, separating the liquid from the solids, but reserve both. Transfer both to the refrigerator and cool 2 hours. Remove the separated cooled soup ingredients from the refrigerator and recombine. Puree the soup in batches in the food processor or in a blender, and add the sorrel leaves a little at a time. The soup will have some texture. When finished processing, taste for seasoning and adjust as needed. Refrigerate until ready to use. Transfer the soup to chilled bowls and garnish with a spoonful of crème fraîche, a drizzle of olive oil, and a few thin slices of leek tops or green garlic tops.