1. Peel and thinly slice the onions from top to bottom so they cook evenly.

2. In a large pot or Dutch oven, melt the butter with the olive oil over medium heat.

3. Add the onions and a pinch of salt, then stir well to coat them in the fat.

4. Cook the onions slowly for 35 to 45 minutes, stirring often, until they become soft, deeply golden, and caramelized.

5. Stir in the minced garlic and cook for 30 seconds.

6. Sprinkle in the flour and stir for 1 minute to coat the onions.

7. Pour in the white wine and scrape the bottom of the pot to loosen the browned bits.

8. Let the wine simmer for 2 to 3 minutes.

9. Add the beef broth, Worcestershire sauce, thyme, bay leaf, salt, and black pepper.

10. Bring the soup to a gentle simmer and cook for 20 to 25 minutes.

11. While the soup simmers, toast the baguette slices until crisp and lightly golden.

12. Remove the bay leaf and thyme stems from the soup.

13. Ladle the soup into oven-safe bowls.

14. Top each bowl with toasted baguette slices, shredded Gruyère, and a little Parmesan.

15. Broil for 2 to 4 minutes until the cheese is melted, bubbling, and lightly browned.

16. Let the bowls cool slightly, then serve hot.