A classic French soup made with caramelized onions, rich beef broth, and topped with toasted bread and melted cheese.

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In a large pot, melt butter with olive oil over medium heat.
Add sliced onions, sugar, and salt. Cook, stirring frequently, for about 30-40 minutes until onions are deeply caramelized.
Add garlic and cook for another minute.
Pour in white wine (if using) and deglaze the pan, scraping up any browned bits.
Add beef broth, bay leaf, and thyme. Bring to a boil, then reduce heat and simmer for 20 minutes. Season with salt and pepper to taste.
Meanwhile, preheat the oven to 400°F (200°C). Toast baguette slices until crisp.
Ladle soup into oven-safe bowls. Place toasted baguette slices on top of soup and sprinkle generously with Gruyère cheese.
Place bowls on a baking sheet and broil in the oven until cheese is bubbly and golden, about 3-5 minutes.
Carefully remove from oven and serve hot.