- Heat the olive oil and butter in a large pot over medium heat. 
- Add the sliced onions and sugar, stirring occasionally until onions are caramelized and golden brown (about 25-30 minutes). 
- Add the minced garlic and cook for another 2 minutes. 
- Sprinkle the flour over the onions, stir well, and cook for an additional 2-3 minutes. 
- Pour in the white wine to deglaze the pot, then add in the beef broth, salt, and black pepper. 
- Reduce the heat and simmer for 30-40 minutes. 
- While the soup simmers, preheat the oven broiler, and toast the baguette slices until golden brown. 
- Ladle the soup into oven-safe bowls, top each with a slice of toasted baguette, and generously sprinkle with Gruyère and Parmesan cheese. 
- Place the bowls under the broiler until the cheese is melted and bubbly.