Preparation Steps

• In a large heavy-bottomed pot or Dutch oven, heat butter and olive oil over medium heat until butter is melted. Add the thinly sliced onions and kosher salt, stirring to coat.
• Cook the onions, stirring frequently, until they soften, about 10 minutes. Sprinkle in the granulated sugar and continue cooking over medium-low heat, stirring occasionally, until the onions are deeply golden and caramelized, about 25 to 30 minutes.
• Increase heat to medium-high and pour in the dry white wine to deglaze the pan, scraping up browned bits from the bottom. Simmer until the wine has mostly evaporated, about 2 to 3 minutes.
• Add the beef broth, fresh thyme leaves, and bay leaf. Bring the soup to a gentle simmer, then reduce heat and simmer uncovered for 15 to 20 minutes to meld flavors. Taste and adjust seasoning with salt and freshly ground black pepper.
• While the soup simmers, preheat the broiler. Arrange the baguette slices on a baking sheet and toast under the broiler until golden on both sides, about 1 to 2 minutes per side.
• Ladle the hot soup into ovenproof bowls. Float one or two toasted baguette slices on each bowl and top generously with grated Gruyère cheese.
• Place the bowls on a baking sheet and put under the broiler until the cheese is melted, bubbly, and lightly browned, 2 to 4 minutes—watch carefully to avoid burning.
• Carefully remove the hot bowls from the oven, discard the bay leaf, garnish with additional thyme if desired, and serve immediately.