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1. **Caramelize the Onions:**
In a large pot, melt the butter with olive oil over medium heat. Add the sliced onions and stir to coat them. Cook the onions for about 30-40 minutes, stirring frequently, until they are deep golden brown and caramelized. Add the garlic during the last few minutes. If desired, add a pinch of sugar to help with caramelization.
2. **Deglaze the Pot:**
Pour in the white wine (if using) and stir, scraping up the brown bits from the bottom of the pot. Let it simmer for a minute to reduce slightly.
3. **Simmer the Soup:**
Add the beef broth and thyme, then bring the soup to a boil. Reduce the heat to low and let it simmer uncovered for 20-30 minutes. Season with salt and pepper to taste.
4. **Prepare the Toppings:**
While the soup is simmering, preheat the broiler. Toast the baguette slices until golden and crispy. Top each slice with a generous amount of grated Gruyère or Swiss cheese.
5. **Serve:**
Ladle the soup into oven-safe bowls and place the cheesy baguette slices on top of each bowl. Broil the bowls in the oven for 2-3 minutes, or until the cheese is melted and bubbly.
Enjoy your classic and comforting French Onion Soup!