Slice the onions thinly. In a large pot or Dutch oven, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, until deeply caramelized and tender, about 25-30 minutes.

Pour in the beef broth and dry white wine, bringing the mixture to a simmer. Season with salt and pepper to taste and let it cook for about 15 minutes to allow the flavors to meld.

While the soup simmers, preheat your broiler. Arrange the baguette slices on a baking sheet and toast them lightly. Sprinkle an even layer of shredded Gruyère cheese over each toasted slice.

Ladle the hot soup into oven-safe bowls. Place a cheese-topped baguette slice on top of each bowl. Broil for 2-3 minutes, or until the cheese is bubbly and golden brown.