In a large Dutch oven or heavy-bottomed pot, melt the butter and olive oil over medium heat. Add the sliced onions and sugar, stirring to coat. Cook for 45-60 minutes, stirring occasionally, until the onions are deeply caramelized, jammy, and rich brown in color. Be patient, as this develops the primary flavor.

Increase heat to medium-high. Pour in the dry sherry to deglaze the pot, scraping up any browned bits (fond) from the bottom. Cook until the sherry has almost completely evaporated. Sprinkle the flour over the onions and cook, stirring constantly, for 1 minute.

Gradually whisk in the beef broth until smooth. Add the thyme sprigs and bay leaf. Bring the soup to a boil, then reduce the heat to low, cover, and let it simmer for at least 30 minutes. Season with salt and pepper to taste. Remove thyme sprigs and bay leaf before serving.

Preheat your broiler. Place the baguette slices on a baking sheet and broil for 1-2 minutes per side until golden and crisp.

Ladle the soup into 6 oven-safe bowls. Top each with a toasted baguette slice and a generous amount of shredded Gruyère cheese. Place the bowls on a baking sheet and broil for 2-4 minutes, or until the cheese is melted, bubbly, and lightly golden. Serve immediately.