In your soup pot, add your butter and cook over medium heat until melted.
Add your sliced onions, chopped garlic, bay leaves, thyme, salt, and pepper. Allow for the mixture to cook over medium-high heat until onions are translucent in color and become caramelized. This typically takes about 20-25 minutes.
Add your wine, and boil for about 5 minutes, or until wine is completely evaporated. Remove and discard bay leaves and thyme sprigs.
Dust flour on top of the onion mixture and gently stir to combine. Turn your burner temperature down to medium-low and cook for 5-10 minutes, to remove the raw flour taste.
Now it is time to add your beef broth to the soup mixture. Turn your heat back up to medium-high and simmer for 10 minutes. Allow to continue to warm, or place into an oven save bowl.
Pour your soup into an oven save bowl and top with baguette slice, and cheese. Place soup into the oven on high broil. Watch it carefully so that it does not burn. Broil for about 2-3 minutes, just until the cheese melts and turns a golden brown. Remove from the oven, and serve while hot. Enjoy!