3 pounds white mushrooms, stems trimmed

3 tablespoons olive oil

3 tablespoons unsalted butter

1 1/2 cups coarsely chopped onion

1 cup coarsely chopped celery (with leaves)

2 tablespoons minced garlic

3/4 teaspoon dried thyme

1/4 teaspoon ground nutmeg

Salt and freshly ground pepper, to taste

6 3/4 cups defatted chicken broth

3/4 cup heavy cream

3 tablespoons snipped fresh chives, for garnish

1. Wipe the mushroom caps clean with a damp paper towel. Slice the mushrooms and set them aside.

2. Heat the oil and butter in a large heavy pot over low heat. Add the onion and celery; cook until wilted, stirring occasionally, 10 to 12 minutes. Add the garlic and cook 2 minutes more.

3. Add the reserved mushrooms and cook uncovered, stirring frequently, for 15 minutes. Season with the thyme, nutmeg, salt and pepper.

4. Add the chicken broth and bring to a boil. Reduce the heat to medium, cover and simmer for 30 minutes. Remove the pot from the heat and let cool to room temperature.

5. Puree the soup in batches in a food processor until smooth, adding the cream gradually through the feed tube. Return the soup to the pot and adjust the seasonings to taste. Before serving, heat the soup through over very low heat (do not boil). Garnish with the chives and serve piping hot.