Grandma's French Onion Soup

4 Tbsp butter
4 cup slice white onions
3 cups beef broth
1 1/2 cups water
1 Tbsp cooking sherry
1 Tbsp Worcestershire sauce
1/4 tsp black pepper
1 dash dried thyme
1 cup soft croutons
1/2 cup shredded mozzarella cheese

Melt butter in large skillet 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 a boil, then reduce heat.  Cover and simmer for 4 minutes.  Divide into bowls.  Top with croutons, then mozzarella.  Broil until cheese melts and begins to brown.