Melt the stick of butter in a large pot over medium heat. Add the onions
, garlic, bay leaves, thyme
, and salt and pepper and cook until the onions are very soft and caramelized, about 25 minutes. Add the wine, bring to a boil, reduce the heat and simmer until the wine
has evaporated and the onions are dry, about 5 minutes. Discard the bay leaves and thyme sprigs. Dust
the onions with the flour and give them a stir. Turn the heat down to medium low so the flour doesn't burn, and cook for 10 minutes to cook out the raw flour taste. Now add the beef broth
, bring the soup
back to a simmer
, and cook for 10 minutes. Season, to taste, with salt and pepper.
When you're ready to eat, preheat the broiler. Arrange the baguette slices on a baking sheet in a single layer. Sprinkle the slices with the Gruyere and broil until bubbly and golden brown, 3 to 5 minutes.
Ladle the soup in bowls and float several of the Gruyere croutons on top.
Alternative method: Ladle the soup into bowls, top each with 2 slices of bread and top with cheese. Put the bowls into the oven to toast the bread and melt the cheese.