Heat a heavy soup pot over medium heat while thinly slicing the onions. Add butter to the pot and melt. Add onions, garlic, bay leaves, thyme, salt and pepper, and gently caramelize, stirring frequently, 30 minutes. If the onions begin to brown to quickly at edges before they caramelize, turn heat down a bit. Deglaze the pan with the wine, let reduce a minute while stirring. Add stock and Worcestershire sauce and simmer 15 minutes more.
Meanwhile, place rack in middle of oven, heat oven to 250˚F.
Place a cooling rack over a baking sheet and arrange the bread slices on it. Toast bread 15 minutes. Switch broiler on. Top the bread with lots of mounded cheese and broil 2-3 minutes to melt cheese.