Melt butter in large saucepan. Add chopped onions and gently saute (about 10 minutes). Add beef broth, celery and potato. Bring to a boil. Cover and simmer for 30 minutes. Puree mixture in a blender. Return to saucepan and add wine, vinegar and sugar. Bring to a boil and simmer 5 minutes. Stir in cream, parsley and pepper to taste. Heat, being sure Not to boil, as the cream will curdle.