In a large saucepan, melt the butter over medium heat. Add the onions and garlic and cook, stirring occasionally, until the onions are golden and caramelized, about 25 minutes.
Add the sugar, beef broth, Worcestershire sauce, thyme, pepper, and salt. Bring to a boil, then reduce the heat and simmer for 10 minutes.
Preheat the oven to 350°F. Place 4 oven-safe soup bowls on a baking sheet. Divide the soup among the bowls and top each with croutons and cheese. Bake for 15 minutes or until the cheese is melted and bubbly.
Serve hot.
Interesting Facts
The origin of French Onion Soup is uncertain, but it is believed to have first appeared in the 18th century.
The cheese traditionally used for this soup is Gruyère, but other cheeses can be substituted, including Swiss, Emmental, or Comté.
This soup can be made ahead of time and reheated when ready to serve.