In a large pot or Dutch oven, I melt the butter and olive oil over medium heat. I add the thinly sliced onions and cook them, stirring occasionally, until they become soft and golden brown. This process takes about 30-40 minutes. To aid in caramelization, I sprinkle the onions with sugar midway through cooking.

Once the onions are caramelized, I stir in the minced garlic and cook for an additional minute. Then, I pour in the dry sherry or white wine to deglaze the pot, scraping up any browned bits from the bottom to enhance the flavor.

I add the beef broth, fresh thyme sprigs, and bay leaf to the pot. Bringing the mixture to a simmer, I let it cook for 20-30 minutes to allow the flavors to meld together. Before serving, I season the soup with salt and freshly ground black pepper to taste and remove the thyme sprigs and bay leaf.

While the soup simmers, I preheat the broiler. I arrange the baguette slices on a baking sheet and toast them under the broiler until they are lightly golden.

I ladle the hot soup into oven-safe bowls, place a toasted baguette slice on top of each, and sprinkle generously with shredded Gruyère cheese. Placing the bowls under the broiler for 2-3 minutes, I wait until the cheese is bubbly and golden brown. I serve the soup hot, savoring its comforting goodness.