Preparation Steps

• Melt butter and olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add sliced onions, salt, and pepper. Cook, stirring occasionally, until onions are softened and translucent, about 10-15 minutes.
• Reduce heat to medium-low and continue cooking the onions, stirring frequently, until they are deeply caramelized and a rich brown color, about 45-60 minutes. Be patient; this slow caramelization is key to the flavor.
• Sprinkle flour over the caramelized onions and stir well. Cook for 1-2 minutes, stirring constantly, to cook out the raw flour taste.
• Gradually whisk in the beef broth and sherry (if using). Add the thyme sprigs and bay leaf. Bring to a simmer, then reduce heat to low, cover, and cook for at least 30 minutes to allow the flavors to meld.
• While the soup simmers, preheat your broiler. Arrange the baguette slices on a baking sheet and broil until lightly golden on both sides. Rub one side of each toasted baguette slice with the halved garlic clove.
• Remove thyme sprigs and bay leaf from the soup. Taste and adjust seasoning with salt and pepper if needed. Ladle the hot soup into oven-safe bowls.
• Place a garlic-rubbed baguette slice on top of the soup in each bowl. Generously sprinkle the shredded Gruyere cheese over the baguette. Place the bowls on a baking sheet and broil for 1-2 minutes, or until the cheese is melted and bubbly and lightly browned.
• Serve immediately, being careful as the bowls will be hot.