In a large pot, melt the butter and olive oil over medium heat. Add the sliced onions, sugar, and salt. 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 beef broth and white wine. Add the thyme, bay leaf, and Worcestershire sauce. Bring to a simmer and let cook for 20-30 minutes. Season with salt and pepper to taste.
Preheat your oven to 400°F (200°C).
While the soup is simmering, arrange the baguette slices on a baking sheet. Toast in the oven for about 5-7 minutes until golden brown.
Remove the bay leaf from the soup. Ladle the soup into oven-safe bowls, top each with a toasted baguette slice, and sprinkle generously with Gruyère cheese.
Place the bowls on a baking sheet and bake in the oven for 10-15 minutes, or until the cheese is bubbly and golden brown.
Carefully remove from the oven and let cool slightly before serving.