​Caramelize the Onions:

In a large pot or Dutch oven, melt the butter with the olive oil over medium heat.
Add the sliced onions, sugar, and salt. Stir to coat evenly, then cook the onions, stirring occasionally, for about 30-40 minutes until they turn golden brown and caramelized.
Adjust the heat as needed to prevent burning, and stir often.
Add Garlic and Deglaze:

Add the minced garlic and cook for an additional 1-2 minutes until fragrant.
Pour in the white wine to deglaze the pot, scraping up any browned bits from the bottom. Simmer for about 5 minutes until the wine is mostly reduced.
Add Broth and Seasoning:

Add the beef broth, bay leaf, and thyme.
Bring the soup to a simmer and let it cook for 20-30 minutes to develop the flavors.
Season with salt and pepper to taste.

Prepare the Baguette Croutons:

Preheat the oven broiler.
Place the baguette slices on a baking sheet and toast them under the broiler until golden brown, about 1-2 minutes per side.
Sprinkle grated Gruyere cheese over each baguette slice and broil until the cheese is melted and bubbly.

Serve the Soup:

Remove the bay leaf from the soup.
Ladle the hot soup into oven-safe bowls, place one or two cheesy baguette slices on top, and, if desired, sprinkle extra cheese.
Broil the soup bowls in the oven for 3-4 minutes until the cheese is bubbly and golden brown.
Enjoy:

Carefully remove the bowls from the oven, let them cool slightly, and enjoy this classic French delight!