In a large soup pot, melt the butter over medium heat. Add the sliced onions and sugar, cooking until the onions are caramelized, about 30 minutes.

Add the beef broth, thyme, bay leaf, salt, and pepper. Bring to a simmer and cook for an additional 15 minutes.

Preheat the oven to 350°F (175°C).

Ladle the soup into oven-safe cups or bowls. Top each with a slice of toasted baguette and a generous amount of Gruyère cheese.

Place the cups on a baking sheet and bake in the preheated oven until the cheese is bubbly and golden, about 10-15 minutes.