In a saucepan over medium heat, melt the butter.
Add the onions and salt and cook until onions start to turn golden brown, stirring occasionally, about 30 minutes.
Add the garlic, sugar, thyme and bay leaf and cook an additional 10 minutes.
Add the stock and worcestershire sauce and bring to a boil. Reduce heat to low and simmer until the onions are very tender, about 10 minutes.
Heat the oven to broil. Place the bread cubes on a baking sheet and broil for 3-5 minutes, or until crisp and golden.
Ladle the soup into oven-proof bowls. Top each bowl of soup with a handful of croutons and a handful of cheese.
Place under the broiler until the cheese is melted, about 2 minutes.