Cream of Mushroom Soup

3 tablespoons unsalted butter
2 tablespoons olive oil
3/4 cup finely chopped shallots
2 pounds white cultivated mushrooms, thinly sliced
1 teaspoon fresh thyme leaves
Salt and freshly ground black pepper
1/3 cup cream Sherry
3 tablespoons all-purpose flour
3 1/2 cups low-salt chicken broth, reserving 1/2 cup
1/2 cup milk
Ground nutmeg to taste, about 1/4 teaspoon
1/2 cup sour cream

In a large saucepan melt butter with the olive oil over moderate heat. Add the shallots and cook until tender, about 2 to 3 minutes. Add the sliced mushrooms, thyme leaves, salt and pepper, cooking until all of the liquid rendered from the mushrooms evaporates. Add the Sherry and cook until evaporated. Remove a 1/2 cup of the cooked mushrooms and reserve. Stir in the flour and cook for 2 to 3 minutes. Add 3 cups of the chicken broth, bring to a simmer and cook for 15 minutes. In a blender puree the soup in batches until smooth. For a very smooth soup, pass through a fine sieve. Return soup to saucepan, bring to a simmer and whisk in the milk. Add reserved chicken broth to achieve desired consistency. Adjust seasoning with salt, pepper

and nutmeg. Lower heat and whisk in 1/4 cup of the sour cream. Keep soup warm but do not simmer or boil. Serve soup garnished with reserved cooked mushrooms and a dollop of the remaining sour cream.