Combine first 3 ingredients in a heavy saucepan; bring to a boil over medium heat. Reduce heat, and simmer 1 hour. Set aside.
Stir together fish bouillon cubes and 2 cups boiling water until bouillon dissolves.
Melt butter in a large, heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add hot fish broth; cook over medium heat until thickened. Stir in cream mixture, and cook until thoroughly heated. Add lemon juice, nutmeg, and crabmeat. Ladle into individual serving bowls. Garnish, if desired. Add a spoonful of sherry to each serving, if desired.
Note: For testing purposes only, we used Knorr Fish Bouillon. It is important to use good-quality sherry, not cooking sherry, for this soup.