Melt the butter in a large Dutch oven over medium heat. Add the onions and garlic and cook, stirring occasionally, for 15 minutes, until the onions are golden brown.
Stir in the beef broth, Worcestershire sauce, sugar, pepper, thyme, and wine. Bring the soup to a boil, then reduce the heat and simmer for 10 minutes.
Preheat the oven to 350°F. Place the toasted bread slices on a baking sheet and top each slice with the shredded cheese. Bake for 5 minutes, until the cheese is melted and bubbly.
Ladle the soup into four oven-safe bowls and top each bowl with a toasted cheese crouton. Place the bowls on a baking sheet and bake for 10 minutes, until the cheese is golden brown and bubbly.
Interesting Facts
French onion soup is believed to have originated in the 18th century.
The cheese traditionally used in French onion soup is Gruyere.
The use of toasted croutons in French onion soup is a relatively recent addition.