Soak dried mushrooms in enough hot water to cover them for 10 to 20 minutes. While soaking, clean and chop the leeks, garlic, potatoes, and fresh mushrooms. Remove the rehydrated mushrooms from the water, reserving the soaking liquid, and chop. Strain the soaking liquid through a mesh strainer lined with cheesecloth or a paper towel.

Heat olive oil over low heat in a soup pot and add leeks, garlic, and potatoes. Cover and sweat until tender, stirring occasionally. After about 8 to 10 minutes, raise heat to medium, add all the mushrooms, and saut For 5 minutes. Sprinkle mushrooms with flour and pour in the sherry, stirring constantly for 4 minutes. Add the mushroom soaking liquid, chicken broth, and thyme. Bring to a boil, reduce heat and simmer, about 20 minutes.

Use an immersion blender to puree the soup to a thick liquid with small bits of mushrooms, or puree the soup carefully and briefly in a blender or food processor.

Stir in yogurt and salt and pepper to taste. Remove from heat and serve.