Melt the butter with the olive oil in a large, wide, heavy-bottomed pot or Dutch oven over medium heat. Add the sliced Vidalia onions and kosher salt, then cook for 10–15 minutes, stirring frequently, until softened and moist.

Reduce the heat to medium-low and continue cooking the onions for 35–50 minutes, stirring periodically. Add the optional sugar if desired, and keep cooking until the onions are deeply golden brown with a rich mahogany color.

Stir in the minced garlic and cook for about 1 minute, just until fragrant. Pour in the wine or sherry and scrape the bottom of the pot to loosen the browned onion bits.

Simmer the wine for 2–3 minutes, then add the beef broth, thyme, bay leaf, and Worcestershire sauce if using. Bring the soup to a simmer and cook uncovered for 20–30 minutes.

Remove the bay leaf and taste the soup, adjusting salt and pepper as needed. Toast the baguette slices until crisp while the soup finishes simmering.

Ladle the hot soup into ovenproof bowls and arrange 1–2 toasted baguette slices over each serving, depending on the bowl size. Cover generously with Gruyere and add Parmesan if desired.

Place the bowls on a sturdy baking sheet and broil for 2–4 minutes, watching closely, until the cheese melts, bubbles, and browns. Serve immediately with extreme care because both the bowls and soup will be very hot.