Slice onions ¼” thick. Cook onions, stirring occasionally, over low heat in melted butter (with sugar if using) until golden, about 30-45 minutes.
Add beef broth, wine, thyme, bay leaf, black pepper, Worcestershire, dry sherry. Bring to a boil, reduce heat and simmer for 1 hour. Remove bay leaf and thyme (if fresh) and discard.
Meanwhile, slice bread and brush with olive oil. Broil 2 minutes per side or until golden.
Ladle soup into ceramic bowls. Add 2 slices of bread to each bowl. Divide cheeses over bowls and broil until golden and bubbly.
