01 -
In a large heavy-bottomed saucepan, melt butter with olive oil over medium heat. Add sliced onions and cook, stirring gently, until onions are deeply golden and caramelized, about 15 to 20 minutes.
02 -
Pour in the white wine, scraping up any browned bits from the base of the pan. Add thyme sprigs and bay leaf. Stir to combine.
03 -
Pour in hot beef broth and season with salt and pepper. Bring the mixture to a gentle simmer and cook uncovered for 20 minutes, allowing flavors to meld.
04 -
While the soup is simmering, toast baguette slices on a baking sheet until crisp and golden.
05 -
Ladle the soup into heatproof bowls. Place a toasted baguette slice on top of each portion and sprinkle generously with grated Gruyère. Broil until the cheese is melted and bubbling.
06 -
Carefully remove hot bowls from the broiler. Serve immediately, garnished with additional thyme if desired.