Instructions:
- Melt butter in a large pot over medium-low heat. Add onions and cook for 30 minutes, stirring regularly, until deeply caramelized and golden brown.
- Pour in the white wine and scrape the bottom of the pot. Cook for 3 minutes.
- Add beef broth and thyme. Season with salt and pepper. Simmer for 15 minutes.
- Ladle soup into oven-safe bowls. Place a baguette slice on top and cover generously with gruyère.
- Broil in the oven for 3–5 minutes until cheese is melted and bubbly. Serve immediately.
