Make the SoupHeat the butter and oil together in a saucepan over low heat; stir in the onion and leek, add a pinch of salt, cover, and allow to sweat for about 5 minutes. Add the asparagus and cook over low heat for about 5 minutes more; add the tarragon and parsley and stir well. Add the broths, bring to a simmer, reduce the heat slightly and simmer for about 20 minutes, or until the asparagus are very tender. Turn off the heat and let sit, covered, for another 20 minutes.
While the soup is cooking, bring a small pan of water to a boil, add salt. Add the asparagus tips and cook until tender, about 2 - 3 minutes. Immediately plunge into an ice bath to stop the cooking and retain the bright vibrant green. Pat dry and refrigerate in some paper towels inside a small plastic bag or container.
Step 3Once the soup has sat, process in batches in a blender until smooth. you may not need all the liquid you cooked in, blend with some of the liquid in each batch, withholding about a cup in reserve. When the soup is ready to be served the next day, use the reserved liquid to bring the soup to the consistency you desire. If you don't have a Vitamix which will make the soup very silky, push the soup through a fine sieve to remove any fibrous pieces of stalk. Place in a container and refrigerate, covered, until chilled, overnight preferred. Refrigerate the reserved cooking liquid separately.
Make the Creme Fraiche: In a small bowl, combine the creme fraiche along with the lemon juice and zest. If you want a thinner sauce, add a little cream. Transfer to a bottle for easy squirting, or you can spoon a dollop on at serving time.
When Ready to ServeRemove the soup from the refrigerator and stir in the cream; taste and adjust salt and pepper. (as stated above, if the soup seems too thick, add some of the reserved cooking liquid) Squeeze a bit of the creme fraiche; garnish with the reserved asparagus tips and a sprinkle of the fresh dill, if using.