Bring the broth and 3 cups of water to a boil in a medium saucepot. Remove from the heat, add the dried mushrooms and set aside to steep for about 20 minutes. Using a slotted spoon, remove the mushrooms and reserve the broth. Roughly chop the mushrooms.
Heat the butter in a large pot over medium-high heat. Add the garlic, white mushrooms and three-quarters of the chopped porcini and cook, stirring occasionally, until softened and mostly dry, about 6 minutes. Add the thyme, shallot and leek and cook, covered, stirring occasionally, until softened but not browned, about 4 minutes. Stir in the flour and cook, stirring, for 2 minutes more.
Strain the reserved mushroom broth into the pot and bring to a boil, whisking constantly. Add the bay leaf, reduce the heat to maintain a simmer and cook for 10 minutes, stirring occasionally. Remove from the heat and cool slightly.
Remove and discard the bay leaf. Working in batches, transfer the mixture to a blender and carefully puree until smooth, taking care not to fill the carafe more than half full per batch. Strain the soup, return to the pot and reheat over medium heat.
Whisk in the cream, sherry, 1 teaspoon salt and several grinds of pepper. Ladle into bowls and sprinkle with the remaining chopped porcini mushrooms and the parsley.
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