Adding bread thickens the soup. Because she enjoys its earthy flavor, Martha adds 1 tablespoon brewer's yeast with the parsley and garlic.
Combine boiling water and porcini in a bowl. Cover and let stand 30 minutes or until tender.
Heat oil in a stockpot over medium-high heat. Add onion; sauté 5 minutes. Add 2 tablespoons parsley and garlic; sauté 5 minutes. Add wine; bring to a boil. Cook 5 minutes. Add portobello and button mushrooms; cook 15 minutes, stirring occasionally.
Add porcini mixture, 4 cups water, broth, and bread; bring to a boil. Reduce heat; simmer 30 minutes. Stir in pepper; let stand 5 minutes.
Place one-fourth of mushroom mixture in a food processor or blender; process until smooth. Pour soup into a large bowl. Repeat procedure with remaining mushroom mixture.
Combine half-and-half and sour cream. Ladle about 2 cups soup into each of 7 bowls. Top each serving with about 1 tablespoon sour cream mixture; sprinkle with about 1 teaspoon parsley.
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