1. Slice the onions thinly into even half-moons.

2. Melt the butter with 1 tablespoon olive oil in a large pot over medium heat, then add the onions and stir well.

3. Cook the onions for 10 minutes until softened, then reduce the heat to medium-low and continue cooking for 30 to 40 minutes, stirring often, until deeply golden and caramelized.

4. Stir in the garlic and cook for 1 minute.

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

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

7. Bring to a gentle simmer and cook uncovered for 20 to 25 minutes.

8. While the soup simmers, brush the baguette slices with olive oil and toast at 400°F for 5 to 7 minutes until crisp and golden.

9. Remove the bay leaf from the soup and ladle the hot soup into oven-safe bowls.

10. Top each bowl with toasted baguette slices, Gruyere, mozzarella or Swiss, and Parmesan.

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

12. Garnish with parsley and crushed red pepper flakes, then serve immediately.