Methods/steps

1. In a saucepan cook the leek and the garlic in the butter over moderately low heat, stirring, until the leek is softened
2. Add the asparagus, the broth, and 1/2 cup water, and simmer the mixture, covered, for 10 to 12 minutes, or until the asparagus is very tender. 
3. Purée two thirds of the mixture in a blender until it is very smooth
4. Stir the purée into the mixture remaining in the pan, and whisk in the sour cream and salt and pepper to taste. 
5. Cook the soup over moderately low heat until it is heated through, but do not let it boil