1. Melt the butter in a large pot over medium heat. Add the onions and cook until caramelized, about 20-25 minutes.

2. Stir in the flour and cook for 2 minutes. Gradually whisk in the beef broth and bring to a simmer.

3. Simmer for 30 minutes, then season with salt and pepper. Preheat the oven broiler.

4. Ladle the soup into oven-safe bowls, top with baguette slices and cheese. Place under broiler until cheese is melted and bubbly.