Heat a heavy soup pot over medium heat while thinly slicing the onions. Add the butter to the pot and melt. Add the onions, garlic, bay leaves, thyme, salt and pepper and gently caramelize, stirring frequently, for 30 minutes. If the onions begin to brown too quickly at the edges before they caramelize, turn the heat down a bit. Deglaze the pan with the wine, then let reduce for 1 minute while stirring. Add the stock and the Worcestershire sauce and simmer for 15 minutes more.
Meanwhile, place an oven rack in middle of the oven and heat the oven to 250˚F.
Place a cooling rack over a baking sheet and arrange the bread slices on it. Toast the bread for 15 minutes. Switch the broiler on. Top the bread with lots of mounded cheese and broil for 2-3 minutes to melt the cheese.
Ladle the soup into shallow bowls and stir in a tablespoon of sherry per bowl, if desired. Set four cheesy croutons into each bowl and serve.