Cut off asparagus tips in about 1/2 inch lengths and reserve.

Cut the soft part of stems into roughly 1/2 lengths.  

In a saucepan cook the leek and the garlic in the butter over moderately low heat, stirring, until the leek is softened.

Add the asparagus stems, the broth, and 1/2 cup water, and simmer the mixture, covered, for 10 to 12 minutes, or until the asparagus is very tender.

Reserve several spoonfulls of the leek, then purée the remaining mixture in a blender until it is very smooth.

Return the puree to the saucepan and add the reserved leek and asparagus tips.

Simmer until asparagus softened.

Whisk in the sour cream and salt and pepper to taste.

Cook the soup over moderately low heat until it is heated through, but do not let it boil.

Add shredded chicken and/or bacon and shredded cheese, if desired.