In a large pot, heat the butter and oil over medium-high heat. Add the onions, sugar, garlic, salt, thyme, and pepper. Cook, stirring occasionally, for 20 minutes, or until the onions are golden brown.
Add the flour and cook, stirring, for 1 minute.
Add the beef broth, wine, bay leaves, and brandy, if using. Bring to a boil, reduce the heat to low, and simmer for 15 minutes.
Ladle the soup into oven-safe serving bowls. Top each bowl with a slice of toasted baguette and a sprinkle of Gruyere cheese. Place the bowls on a baking sheet and broil for 2 to 3 minutes, or until the cheese is melted and bubbly.
Serve hot.
Interesting Facts
French Onion Soup is a classic French dish that is perfect for cold winter days.
The key to a good French Onion Soup is to caramelize the onions until they are golden brown.
Gruyere cheese is the traditional cheese used for French Onion Soup, but you can also use Swiss or Gruyere-style cheese.