Nothing says Spring like asparagus, and this light soup makes a lovely seasonal starter or an elegant addition to a special lunch. To preserve the soup's pretty green color, don't allow the blanched asparagus to cook in the final stages for more than the specified time.
Steam the asparagus in a vegetable steamer until crisp-tender, 2 to 4 minutes depending on the thickness of the stalks. Immediately plunge the asparagus into a bowl of ice water. When cool, drain and set aside, reserving 8 asparagus tips for garnish.
Melt the butter in a large pot over low-medium heat. Add the leeks and cook, stirring occasionally, until the leeks are tender, 10 to 15 minutes. Add the potatoes and cook, stirring once or twice, for 5 minutes.
Add the chicken stock and raise the heat to high. Bring the soup to a boil, then reduce the heat to medium, cover the pan, and cook until the potato is very tender, about 20 minutes.
Add the asparagus (except for the reserved tips) and cook, uncovered, for 2 minutes.
Purée the soup in a blender, food processor, or with an immersion blender. Strain through a fine mesh strainer. If the soup has cooled, reheat gently until hot. Garnish each serving with 2 reserved asparagus tips.