2. Turn the heat to high and brown the onions, stirring frequently to keep them from burning, about 5 minutes. Add the garlic and cook for 1 more minute.
3. Add the sherry (if desired), thyme and bay leaf. Cook until the sherry has almost completely evaporated.
4. Add the beef broth and simmer for 10 minutes. Season with salt and pepper.
5. Preheat the broiler to medium-high.
6. Lay the baguette slices on a baking sheet and place about 2 teaspoons of cheese on each slice. Broil until the cheese is melted and the baguette is browned on the edges.
7. Ladle the soup into bowls and garnish with a cheese crouton.