In a large pot over medium-high heat, melt the butter. Add the onions, garlic, thyme, bay leaf, salt and pepper and cook for about 40 minutes or until the onions are soft and golden brown, stirring occasionally.
Sprinkle the flour over the onion mixture and cook for another 5 minutes, stirring constantly.
Add the broth, wine and Worcestershire sauce and bring the soup to a boil. Reduce the heat and simmer for 15 minutes.
Discard the thyme sprigs and bay leaf.
Preheat the oven to 375°F.
Cut the bread into slices and toast them on a baking sheet for 10 minutes or until crisp.
Ladle the soup into four oven-safe bowls and place them on a baking sheet. Top each bowl with two slices of bread and some cheese.
Bake for 10 minutes or until the cheese is melted and bubbly.
Garnish with parsley and serve hot.