In a large heavy-bottomed pot or Dutch oven, I melt the butter with the olive oil over medium heat. Adding the thinly sliced onions, I stir to coat them evenly. Sprinkling in the sugar helps to enhance the caramelization process. I cook the onions, stirring occasionally, until they become soft and develop a deep golden brown color. This process takes about 30 to 40 minutes but is crucial for developing the soup's rich flavor.

Once the onions are beautifully caramelized, I stir in the minced garlic and cook for an additional minute. Pouring in the dry sherry or white wine, I deglaze the pot, scraping up any browned bits stuck to the bottom. This step adds depth to the soup's flavor.

Next, I pour in the beef broth and add the thyme and bay leaf. Bringing the mixture to a simmer, I let it cook for about 20 to 30 minutes to allow the flavors to meld together. I then season with salt and freshly ground black pepper to taste.

While the soup simmers, I preheat the broiler. Placing the baguette slices on a baking sheet, I toast them under the broiler until they're lightly golden. This usually takes just a few minutes, so I keep a close eye to prevent burning.

Once the soup is ready, I remove the bay leaf and ladle the hot soup into oven-safe bowls. I place a toasted baguette slice on top of each bowl and generously sprinkle the shredded Gruyère cheese over the bread. Placing the bowls under the broiler, I heat them until the cheese is bubbly and golden brown. Carefully removing the bowls from the oven, I serve the soup immediately, enjoying the comforting blend of flavors and textures.