Place dried shiitake mushrooms in a small bowl; cover with boiling water. Let stand 30 minutes. Drain, rinse and chop mushrooms, discarding stems. Set aside.Advertisement
In a 4-quart Dutch oven, cook onion and garlic in hot olive oil over medium heat until onion is tender but not brown. Add rice and thyme; cook and stir for 2 minutes. Add shiitake, button and portabello mushrooms; cook and stir 2 minutes more. Add broth; bring to boiling. Reduce heat and simmer, covered, for 1 hour, stirring occasionally. Remove from heat; cool slightly (5 to 10 minutes).
Transfer, about half at a time, to blender or food processor. Cover and blend or process until smooth. Season to taste with salt and pepper. Top with parsley and/or sour cream, if desired.