Place dried mushrooms in a small bowl. Add the 1/2 cup broth and the wine. Let stand 30 minutes. Drain and discard the excess liquid. Coarsely chop mushrooms; set aside.
Melt butter in large saucepan over medium heat. Add onion; cook and stir for 2 to 3 minutes still tender. Stir in fresh and dried mushrooms; cook 3 to 4 minute more or till most liquid has evaporated. Stir in flour. Add the 8 cups broth and pepper. Cook and stir to boiling. Reduce heat; cook and stir, uncovered, for 30 minutes more.
Remove 4 cups soup mixture; puree in blender or food processor. Return to saucepan. Stir in buttermilk and heat through. Do not boil.