Step 1

Heat oil in a large heavy saucepan or stockpot over low heat. Add leeks and cook, stirring, until softened, about 10 minutes. Add garlic and thyme; cook for 2 minutes more. Pour in broth, increase heat to medium and bring to a boil. Reduce heat to low and simmer, uncovered, for 10 minutes.

Step 2

Pour the soup through a strainer set over a large bowl. Puree the leeks in a food processor or blender until smooth, adding some the broth if necessary. Return the puree and broth to the saucepan. Add potatoes and simmer, covered, until the potatoes are soft, 10 to 15 minutes. Remove from the heat and mash the potatoes thoroughly with a potato masher.

Step 3

Stir in sour cream, salt and pepper. Return to low heat and heat until hot, but not boiling. Serve hot or chilled.