Melt the butter in a large pot over medium heat. Add the onions and cook until very brown and caramelized, about 1 hour. Depending on how large your onions are, this may take more or less time. Keep cooking until they brown, stirring occasionally.

Add the garlic, bay leaf, thyme and mushrooms and cook another 10-15 minutes, until mushrooms are browned.

Add the wine, bring to a boil, then reduce heat to a simmer until liquid is evaporated.

Remove the bay leaf and thyme stems. Sprinkle on flour and stir. Reduce heat to low and cook another 10 minutes. Add the broth, bring to a simmer and cook for at least 20 minutes.

When ready to serve, preheat the broiler. Divide the soup into 4-6 bowls and top each with 1-2 slices of bread. Sprinkle with gruyere cheese and broil for 3-5 minutes, until golden brown.