Place flour in a small skillet over medium-high heat; cook 2 minutes or until flour turns light brown, stirring constantly. Transfer the flour to a small plate; cool.
Combine boiling water and porcini mushrooms in a bowl; cover and let steep 20 minutes. Strain porcini mushrooms through a sieve over a bowl, reserving the soaking liquid. Chop the mushrooms.
Melt butter in a Dutch oven over medium-high heat. Add onion; sauté 5 minutes or until tender. Add garlic, thyme, salt, and pepper; sauté 30 seconds. Add shiitake mushrooms; cook 3 minutes, stirring constantly. Add button mushrooms; cook 3 minutes, stirring constantly. Add chopped porcini mushrooms and broth to pan.
Combine reserved porcini soaking liquid with toasted flour, stirring with a whisk, and add to pan. Bring to a boil; reduce heat, and simmer, uncovered, for 15 minutes. Add milk; simmer 10 minutes, stirring frequently. Remove from heat; stir in half-and-half and sherry. Place 2 cups soup in a blender; process until smooth. Return pureed soup to pan. Warm soup over low heat for 5 minutes or until thoroughly heated. Divide soup among each of 8 bowls, and sprinkle with parsley, if desired.