A classic and comforting French Onion Soup recipe with caramelized onions in a rich beef broth, topped with a toasted baguette and melted Gruyere cheese.
Melt butter and olive oil in a large pot or Dutch oven over medium heat. Add onions and cook, stirring occasionally, until softened and caramelized, about 30-40 minutes.
Add beef broth, red wine (if using), thyme, salt, and pepper to the pot. Bring to a simmer and cook for 15 minutes.
Preheat broiler. Ladle soup into oven-safe bowls. Top each bowl with a toasted baguette slice and sprinkle generously with Gruyere cheese.
Broil until cheese is melted and bubbly, about 2-3 minutes. Watch carefully to avoid burning.
Serve immediately.
Notes
For a richer flavor, use homemade beef broth. You can also add a splash of sherry vinegar at the end for brightness.