Steps
1. In a saucepan, melt butter with olive oil over medium heat.
2. Add sliced onions and sugar; cook slowly, stirring occasionally, until onions are deeply caramelized (about 25 minutes).
3. Add minced garlic and cook for 1 minute.
4. Pour in beef broth and white wine, if using; add thyme if desired.
5. Simmer for 15 minutes; season with salt and pepper.
6. Meanwhile, toast the baguette slice until golden.
7. Pour soup into an oven-safe bowl, place toasted baguette on top, and sprinkle with Gruyère cheese.
8. Broil in the oven until cheese is melted and bubbly.
9. Serve hot.