2In a large pot, melt the unsalted butter with the olive oil over medium heat.
Add the thinly sliced onions, sugar, and salt to the pot, stirring to coat the onions with the butter and oil.
Cook the onions, stirring occasionally, until they are caramelized to a deep golden brown, about 30-40 minutes.
Sprinkle the all-purpose flour over the caramelized onions and cook, stirring, for about 2 minutes to remove the raw flour taste.
Slowly pour in the dry white wine, scraping any browned bits off the bottom of the pot.
Add the beef stock, bay leaf, and dried thyme to the pot. Bring the soup to a simmer and cook for another 30 minutes.
Preheat the broiler. Remove the bay leaf from the soup and discard.
Ladle the soup into oven-safe bowls. Place a slice of toasted baguette on top of each bowl of soup.
Sprinkle the grated Gruyère cheese evenly over the slices of baguette.
Place the bowls under the broiler and broil until the cheese is bubbly and golden brown, about 3-5 minutes.
Serve the French onion soup hot, with extra toasted baguette slices if desired.