In a large pot over medium heat, combine the butter and olive oil. Once melted, add the sliced onions. Sprinkle with sugar to help them caramelize and cook for about 30-40 minutes, stirring frequently, until they are golden brown.

Pour in the dry white wine to deglaze the pot, lifting the caramelized bits stuck at the bottom. Allow the wine to reduce for about 5 minutes.

Pour in the beef broth and add thyme. Season with salt and pepper. Bring to a simmer and let it cook for at least 30 minutes to meld the flavors.

While the soup simmers, preheat your oven to 400°F (200°C). Arrange the sliced baguette on a baking sheet and toast until golden brown.

When you're ready to serve, ladle the soup into oven-safe bowls. Place a couple of toasted baguette slices atop each bowl and generously sprinkle with Gruyère cheese.

Place the bowls in the preheated oven until the cheese is bubbly and golden, about 5-10 minutes, watching carefully to avoid burning.