1. Caramelize the Onions

• Heat butter and olive oil in a large, heavy-bottomed pot over medium heat.
• Add sliced onions and stir to coat them evenly with the butter and oil.
• Sprinkle with salt and sugar, which will help draw out moisture and enhance caramelization.
• Cook the onions, stirring occasionally, until they become a deep golden brown. This process can take 30-40 minutes but is crucial for developing the soup’s rich flavor.

2. Deglaze the Pot

• Add minced garlic to the caramelized onions and cook for an additional minute until fragrant.
• Pour in the white wine or sherry to deglaze the pot, scraping up any browned bits from the bottom. Allow the liquid to reduce by half, intensifying the flavors.

3. Add Broth and Herbs

• Stir in the beef broth, bay leaf, and thyme.
• Bring the mixture to a boil, then reduce the heat and let it simmer uncovered for about 30 minutes. This allows the flavors to meld together beautifully.
• Season with salt and freshly ground black pepper to taste, keeping in mind the saltiness of your broth.

4. Prepare the Bread Topping

• Preheat your oven’s broiler.
• Arrange baguette slices on a baking sheet and toast them under the broiler until golden brown on both sides.
• Remove from the oven and set aside.

5. Assemble and Serve

• Remove the bay leaf and thyme sprigs from the soup.
• Ladle the hot soup into oven-safe bowls, leaving some space at the top.
• Place a toasted baguette slice on top of each bowl of soup.
• Sprinkle generously with grated Gruyère cheese, covering the bread and the surface of the soup.
• Place the bowls on a baking sheet and transfer them to the oven under the broiler.
• Broil until the cheese is melted and bubbly, with a slight golden brown crust, about 2-3 minutes.
• Carefully remove from the oven (the bowls will be hot) and serve immediately.