Step 1

In a medium saucepan, melt butter over medium heat. Add leeks and shallot; season with salt and pepper. Cook, stirring, until translucent, about 5 minutes. Add asparagus and 3 cups broth; bring to a boil, then reduce heat and simmer, partially covered, until asparagus is very tender, 12 to 15 minutes. Let cool 5 minutes.

Step 2

Working in batches (don’t fill it more than halfway), purée asparagus in a blender until very smooth, 1 to 2 minutes a batch, thinning with more broth if necessary. Transfer to cleaned pan, and heat until steaming. Whisk crème fraîche with a little water until it has the consistency of heavy cream. Spoon some on top of each serving, swirling with a toothpick, if desired.