In a large, heavy bottomed pot or Dutch oven, melt the butter with the olive oil over medium low heat. Add the sliced onions, sugar, salt, and pepper. Cook, stirring occasionally, for 45-60 minutes, until the onions are deeply caramelized, a rich golden brown color, and very soft. This slow caramelization is crucial for flavor development.

Add the minced garlic and cook for 1 minute more until fragrant, being careful not to burn it.

Increase the heat to medium. Deglaze the pot: Pour in the dry sherry. Scrape the bottom of the pot vigorously with a wooden spoon to loosen all the browned bits (fond). Let the sherry simmer and reduce by about half, about 2-3 minutes.

Pour in the beef stock. Add the fresh thyme sprigs and bay leaf. Bring the soup to a gentle simmer. Reduce the heat to low, cover partially, and let it simmer for at least 20-30 minutes to allow the flavors to meld.

While the soup simmers, prepare the croutons. Brush the baguette slices lightly with olive oil. Toast them in a 375°F (190°C) oven or under a broiler until golden brown on both sides. Rub one side of each toasted baguette slice with the cut side of the halved garlic clove.

Remove the thyme sprigs and bay leaf from the soup. Taste and adjust seasoning with salt and pepper if necessary. Ladle the hot soup into oven safe bowls.

Place a garlic rubbed baguette slice on top of each bowl of soup. Generously sprinkle the grated Gruyère cheese over the baguette, ensuring it covers the bread completely.

Place the bowls on a baking sheet and broil for 2-4 minutes, or until the cheese is melted, bubbly, and golden brown. Watch carefully to prevent burning.