In a large pot or Dutch oven, melt the butter and olive oil over medium heat.
Add the sliced onions, sugar, salt, and pepper. Cook, stirring frequently, for about 30-40 minutes until the onions are caramelized and golden brown.
Add the minced garlic and cook for an additional 1-2 minutes until fragrant.
Pour in the white wine (if using) and scrape the bottom of the pot to deglaze, allowing the wine to reduce for about 5 minutes.
Add the beef broth, thyme, and bay leaf. Bring the mixture to a simmer and let it cook for 20-30 minutes to develop the flavors.
While the soup is simmering, preheat your oven to 400°F (200°C).
Place the slices of baguette on a baking sheet and toast them in the oven for about 5-7 minutes until golden brown.
Remove the bay leaf from the soup and ladle the soup into oven-safe bowls. Top each bowl with a slice of toasted baguette and sprinkle generously with Gruyère and Parmesan cheese.
Place the bowls on a baking sheet and broil in the oven for 3-5 minutes until the cheese is bubbly and golden.
Serve immediately, garnished with additional thyme if desired.