Place dried mushrooms in medium bowl and cover with boiling water. Let stand for at least 1 hour.
Heat butter and oil in a medium saucepan and add onion. Saute over low heat, stirring occasionally, for 3 to 5 minutes or until soft. Add fresh mushrooms. Stir and cook until they are softened. Sprinkle with flour, salt, and pepper. Stir for 1 minute to coat the mushrooms, then add veal or beef stock and chicken stock.
Drain dry mushrooms to measure soaking liquid. Strain liquid through cheesecloth to remove grit. Add enough water to make 2 cups and add to saucepan.
Add dried mushrooms and simmer 15 minutes.
Puree half of soup mixture in food processor fitted with steel blade. Repeat with other half.
Return mixture to saucepan, bring almost to boil, then add half and half, sherry, and lemon juice. Taste for seasoning.
To serve, garnish with a dollop of sour cream and chopped parsley.