Cook onions in butter until transparent. Add flour; blend well. Put in slow cooker. Add beef broth and Tabasco. Cook on low 7 to 9 hours or high 4 to 5 hours. Ladle soup into oven-proof bowls. Top each bowl of soup with a slice of bread and shredded Mozzarella cheese. Broil until cheese melts. Sprinkle individual servings with Parmesan cheese, or serve grated Parmesan on the side. Serves 6.