French Onion Soup with Gruyere Croutons

Melt butter and olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions soften and begin to brown, about 15 minutes.

Reduce heat to low and continue cooking, stirring frequently, until the onions are deeply caramelized and a rich brown color, about 45-60 minutes. This is a slow process, but it's essential for the flavor of the soup. Be patient and don't rush it. If the onions start to stick, add a tablespoon of water or broth.

Add the minced garlic and cook for 1 minute more. Stir in the dry sherry or white wine (if using) and deglaze the pot, scraping up any browned bits from the bottom.

Pour in the beef broth, add the thyme sprigs and bay leaf (if using), and bring to a simmer. Season with salt and pepper to taste. Reduce heat to low and simmer for at least 15 minutes to allow the flavors to meld.

While the soup simmers, preheat oven to 375°F (190°C). Place baguette slices on a baking sheet and toast in the oven for about 5-7 minutes, or until lightly golden brown.

Remove the thyme sprigs and bay leaf from the soup. Ladle the soup into oven-safe bowls or ramekins.

Top each bowl with two toasted baguette slices, then generously cover with shredded Gruyere cheese.

Place the bowls on a baking sheet and broil in the oven until the cheese is melted and bubbly, about 2-3 minutes. Watch carefully to prevent burning.

Let the soup cool slightly before serving. Serve immediately.