You may serve this fresh spring soup either hot or cold, which makes it a great option for entertaining.
Bring a large pot of salted water to a boil. Add asparagus and boil until it’s fork-tender and bright green (3–6 minutes, depending on thickness). Drain asparagus and submerge in ice water to stop cooking.
At this point, you may either bring the stock to a boil or keep it cool, depending on whether you want to serve the soup hot or cold. Add asparagus to stock, along with tarragon and lemon juice. Purée in a blender until smooth. Season with salt and pepper to taste.
Meanwhile, add lemon zest to crème fraîche. Serve soup, hot or cold, in small bowls, topped with crème fraîche.