PORTABELLO MUSHROOM BISQUE

1-1/2 ounces dried porcini mushrooms, or other dried mushrooms
7 cups chicken broth, homemade or low-sodium canned
6 tablespoons unsalted butter
5 cloves garlic, minced
10 ounces button mushrooms, chopped
2 leeks (white part only), thinly sliced
2 medium shallots, thinly sliced
7 tablespoons all-purpose flour
3 parsley sprigs
3 sprigs fresh thyme
1 bay leaf
1/2 cup heavy cream
1 teaspoon Madeira or Sherry
1 teaspoon kosher salt
Freshly ground black pepper
Crème fraiche or dairy sour cream
Nutmeg

Place the dried mushrooms in a large bowl. Bring the broth to a boil and pour over the dried mushrooms (white wine and hot water can be substituted for the stock if you like). Set aside to rehydrate for about 20 minutes.
Using a slotted spoon, remove the mushrooms and reserve the liquid. Roughly chop the mushrooms.
Heat the butter in a large pot over medium-high heat. Add the button and rehydrated mushrooms and cook, stirring occasionally, until soft and somewhat dry, about 6 minutes. Add the leek and shallot and cook, covered, stirring occasionally, until translucent, about 4 minutes. Stir in the flour and cook, stirring, for 2 minutes more.
Pour in the reserved mushroom liquid, taking care not to add any grit or dirt that may have fallen to the bottom of the boil, and bring to a boil while whisking constantly. Tie the parsley sprigs, fresh thyme, and bay leaf together with a piece of kitchen twine and add to the soup. Lower the heat and simmer for 10 minutes. Remove from the heat and allow to cool.
Remove and discard the herb bundle. Working in batches, transfer the mixture to a blender and puree until smooth. Using a sieve over a large bowl, strain the mushroom puree. Return the puree to the pot and reheat over medium heat.
Whisk the heavy cream, Madeira or Sherry, and salt into the soup and season with pepper to taste.
Whisk crème fraiche or sour cream together with nutmeg to taste.
Divide among warm soup bowls, garnish with nutmeg crème fraiche and serve immediately.