A rich and savory French onion soup featuring caramelized onions, flavorful broth, and cheesy toasted bread.

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In a large pot over medium heat, melt butter with olive oil. Add onions and cook, stirring often, until very soft and golden brown, about 30-40 minutes.
Add garlic and sugar; cook for 1-2 minutes more.
Stir in flour and cook for another 2 minutes to thicken slightly.
Deglaze the pot with white wine, scraping up any browned bits (optional).
Add beef broth, bay leaf, thyme, salt, and pepper. Bring to a boil, then reduce heat and simmer for 20 minutes.
While soup simmers, toast baguette slices until crisp.
Remove bay leaf from soup. Ladle soup into oven-safe bowls, top with a slice of toasted baguette and a generous sprinkle of Gruyère cheese.
Place bowls under the broiler until the cheese is melted and bubbly, about 2-3 minutes.
Serve hot, being careful with the hot bowls.

Use vegetable broth for a vegetarian version, but the flavor will be slightly different.
Caramelizing the onions slowly is key for the best flavor.
Make the soup ahead and broil with cheese just before serving for easy entertaining.
Gruyère cheese is traditional, but Swiss or mozzarella can be used as substitutes.