In a large pot or Dutch oven, melt the butter with olive oil over medium heat. Add the thinly sliced onions and cook, stirring occasionally, for about 30-40 minutes, or until the onions are caramelized and golden brown.
Once the onions are caramelized, pour in the beef and chicken broth, white wine (if using), Worcestershire sauce, and balsamic vinegar (if using). Season with salt and pepper to taste. Bring the soup to a simmer and let it cook for an additional 15-20 minutes to allow the flavors to meld together.
While the soup is simmering, preheat your broiler. Arrange the sliced baguette or French bread on a baking sheet and toast under the broiler until golden brown.
Ladle the hot soup into oven-safe bowls or crocks. Place a toasted slice of baguette or French bread on top of each bowl of soup.
Sprinkle a generous amount of grated Gruyere or Swiss cheese over the bread slices.
Place the bowls of soup under the broiler and broil for 2-3 minutes, or until the cheese is melted and bubbly and starting to brown.
Carefully remove the bowls from the broiler (they will be hot!) and garnish with fresh thyme or parsley if desired.
Serve the French Onion Soup hot and enjoy the delicious combination of caramelized onions, rich broth, and gooey melted cheese.
This French Onion Soup recipe is perfect for a cozy night in or as an impressive starter for a dinner party. It’s easy to make but tastes like it came from a fancy restaurant!
