Grind mushrooms and onions in a food processor or meat grinder. Melt 1 T. butter in large pan over medium high heat. Add mushroom mixture and cook, stirring constantly, until all water evaporates. Add water and bouillon cubes to mushroom mixture; boil soup for 10 minutes. Meanwhile, combine flour and remaining 2 T. butter until a paste forms. (Beurre Marie). Whisk Beurre Marie into soup, whisking until soup thickens. Add heavy cream to soup and bring it to a boil. Skim top as the soup boils. Season soup heavily with white pepper. Serve soup in bowl accompanied by crusty French bread. Note: 1/2 cup whole milk and 1/2 cup cream can be substitute for the 1 cup cream.