In a large pot, melt the butter and olive oil over medium heat.

Add the sliced onions and sugar, stirring frequently until they become golden brown, about 25 minutes.

Once the onions are caramelized, stir in the salt, black pepper, and thyme.

Deglaze the pot by adding the dry white wine, scraping up any brown bits from the bottom.

Pour in the beef broth and bring the mixture to a simmer.

Allow it to cook for an additional 15 minutes.

While the soup simmers, preheat your oven to 350°F (175°C).

Arrange the baguette slices on a baking sheet and toast them until golden, about 10 minutes.

Divide the soup among oven-safe bowls, placing a toasted baguette slice on top of each.

Generously sprinkle shredded Gruyère cheese over the bread in each bowl.

Place the bowls under the broiler for 3-5 minutes, watching closely until the cheese is bubbly and golden.