1. Heat the butter and olive oil in a large heavy pot over medium heat. Add the sliced onions and salt, then cook for 25 to 30 minutes, stirring often, until very soft and golden brown.

2. Stir in the minced garlic and cook for 30 seconds until fragrant.

3. Sprinkle the flour over the onions and stir well for 1 minute so it coats everything evenly.

4. Slowly pour in the beef broth while stirring, then add the Worcestershire sauce, thyme, and black pepper.

5. Bring the soup to a gentle simmer and cook for 15 to 20 minutes. Taste and adjust seasoning if needed.

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

7. Ladle the soup into oven-safe bowls. Top each bowl with toasted baguette slices, then add Gruyère and mozzarella.

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

9. Let cool for 1 to 2 minutes, then serve hot.