Portobello Mushrooms Soup Recipe

Name: Portobello Mushrooms Soup Recipe
Ingredients: 1/4 cup (1/2 stick) unsalted butter; 5 leeks (white and pale green only) chopped; 1 med onion chopped; 10 ozs. portobello mushrms: chopped to make 4 generous cups; 1/4 cup flour; 3 cups chicken stock or broth; 2 cups half and half; 1/4 tsp cayenne pepper; 1/4 tsp groung white pepper; Optional, 1/2 cup f sour cream