Melt butter in saucepan over low heat. -Add onions and cook 20 minutes, stirring occasionally. -Add beef broth, water, sherry, Worcestershire sauce, pepper, and thyme to pot. -Increase heat and bring to boil, then reduce heat. -Cover and simmer for 5 minutes. -Divide into bowls. -Top with croutons, then mozzarella. -Broil until cheese melts and begins to brown.