In a large pot over medium heat, melt the butter and add the onions. Cook until softened, about 10 minutes.

Add sugar and pepper and cook for an additional 5 minutes until onions are caramelized.

Pour in the broth and white wine (if using) and bring to a boil. Reduce heat and simmer for 30 minutes, stirring occasionally.

Preheat oven to 375 degrees F (190 degrees C). Place French bread slices on a baking sheet and top with shredded cheese. Bake until melted and lightly golden brown, about 10 minutes.

To serve, ladle soup into individual oven-safe bowls and top with the cheese-topped French bread. Place under the broiler until cheese is melted and bubbly, about 5 minutes.

Serve hot and enjoy!