In a large Dutch oven over medium heat, melt the butter. Add the sliced onions and sprinkle with salt and sugar. Cook, stirring occasionally, until the onions become soft and golden brown, about 30 minutes. This slow caramelization process is crucial for developing the soup's deep, sweet flavor.

Pour in the white wine to deglaze the pot, scraping up any browned bits stuck to the bottom. Allow the wine to reduce until it's almost evaporated, intensifying the flavor.

Stir in the beef broth, dried thyme, and bay leaf. Bring the mixture to a boil, then reduce the heat to low. Let it simmer gently for about 30 minutes, allowing the flavors to meld beautifully.

After simmering, taste the soup and adjust the seasoning with salt and freshly ground black pepper as needed. Remove and discard the bay leaf.

While the soup simmers, preheat your oven's broiler. Place the toasted baguette slices on a baking sheet and sprinkle generously with the grated Gruyère cheese. Broil until the cheese is bubbly and golden, keeping a close eye to prevent burning.

Ladle the hot soup into oven-safe bowls. Top each with a cheesy baguette slice. For an extra touch, you can place the bowls under the broiler briefly to melt the cheese further. Serve immediately and enjoy the comforting warmth.