Pour 2 cups of the boiling water over the porcini mushrooms. Let stand 20 minutes. Drain the porcini mushrooms in a colander over a bowl, reserving the mushroom broth; set the mushrooms aside. Strain the mushroom broth through a cheesecloth-lined colander into a bowl; discard the solids. Stir in the cornstarch, soy sauce, salt, pepper, and remaining 2 cups boiling water into the mushroom broth; set aside.

Heat the oil in a medium-sized nonstick skillet over medium-high. Add the shallots and garlic; cook, stirring occasionally, until the shallots are soft, 4 to 5 minutes. Add the sherry; bring to a boil, and cook 30 seconds. Remove from the heat.

Stir together the porcini mushrooms, broth mixture, shallot mixture, fresh mushrooms, and thyme in a 5-quart slow cooker. Cover and cook on HIGH until the vegetables are very tender and the flavors blend, about 4 hours. Uncover and cook until slightly thickened, about 20 minutes. Transfer 2 cups of the soup to a blender. Remove the center piece of the blender lid (to allow steam to escape); secure the lid on the blender. Place a clean towel over the opening in the blender lid (to avoid splatters). Blend until smooth, about 10 seconds. Return the puréed soup to the slow cooker; gently stir in the cream. Ladle the soup into bowls, and serve hot.