In a large pot, melt the butter over medium-high heat.
Add the onions and garlic to the pot, and cook for 5 minutes, or until the onions are softened and lightly browned.
Add the thyme, rosemary, and flour, and cook for 1 minute.
Gradually add the beef broth, red wine, Worcestershire sauce, balsamic vinegar, and bay leaves. Bring to a boil, then reduce the heat to a simmer and cook for 20 minutes.
Taste and season with salt and pepper, as desired.
To serve, ladle the soup into oven-safe bowls. Top each bowl with a slice of toasted French bread and a slice of Swiss cheese. Place the bowls on a baking sheet and broil for 3-5 minutes, or until the cheese is melted and lightly browned.
Serve hot and enjoy!
Interesting Facts
French Onion Soup dates back to the 18th century.
The recipe is believed to have originated in Paris.
French Onion Soup is often served with a side of crusty bread.
The classic version of French Onion Soup is made with beef broth.