Heat the butter in a large pot over medium heat. Add the onions and garlic and cook, stirring occasionally, for 15 minutes, or until the onions are golden and caramelized.
Add the beef broth, bay leaf, thyme, Worcestershire sauce, salt and pepper. Bring to a boil, then reduce the heat and simmer for 15 minutes.
Stir in the flour and simmer for 5 more minutes.
Preheat the oven to 350 degrees F (175 degrees C).
Place the French bread slices on a baking sheet and top each slice with a slice of Swiss cheese. Bake for 10 minutes, or until the cheese is melted and bubbly.
Divide the soup among 4 ovenproof bowls.
Top each bowl of soup with a slice of cheesy French bread and place the bowls on a baking sheet.
Bake for 10 minutes, or until the cheese is golden and bubbly.
Serve hot.
Interesting Facts
French Onion Soup was first created in 1790 in Paris.
It is a popular dish in France, and is often served as a starter before the main course.
Traditionally, the soup is made with beef broth, but it can also be made with chicken or vegetable broth.
The soup is usually topped with a slice of toasted French bread and melted cheese.