In a large Dutch oven or heavy-bottomed pot, melt the butter over medium heat. Add the sliced onions and cook for 25-30 minutes, stirring occasionally, until they are deeply golden brown and caramelized.

Sprinkle flour over the caramelized onions and stir continuously for 2 minutes to cook out the raw flour taste. Slowly pour in the beef broth while stirring to prevent lumps from forming.

Pour in the white wine (if using), then add the bay leaf and thyme. Season with salt and pepper to taste. Bring the soup to a boil, then reduce heat and simmer for 15-20 minutes to allow flavors to meld.

Preheat your broiler. Place oven-safe soup bowls on a baking sheet. Remove the bay leaf and thyme sprigs from the soup.

Ladle the hot soup into the oven-safe bowls. Place 2 baguette slices on top of each soup bowl. Combine the three cheeses in a bowl, then generously sprinkle the cheese mixture over the bread, ensuring some falls over the edges.

Place the baking sheet with filled soup bowls under the broiler for 2-3 minutes, or until the cheese is bubbly, melted and lightly browned. Watch carefully to prevent burning.

Carefully remove from the oven (the bowls will be extremely hot), sprinkle with fresh parsley, and serve your French onion soup while the cheese is still bubbling.