Heat the butter and olive oil in a large pot over medium heat.
Add the sliced onions and stir to coat them in the fat.
Sprinkle sugar over the onions to help with caramelization.
Cook the onions for about 30 minutes, stirring occasionally, until golden brown.
Deglaze the pot with white wine and let it simmer for 5 minutes.
Add the beef broth and thyme, then season with salt and pepper.
Bring the soup to a boil, then reduce heat and let it simmer for 10 minutes.
While the soup simmers, preheat the oven to 400°F (200°C).
Toast the baguette slices until golden brown.
To serve, ladle the soup into bowls, top with toasted baguette slices, and sprinkle with cheese.
Place the bowls under the broiler for 2-3 minutes until the cheese is bubbly and golden.