In a large skillet, melt 2 tablespoons of butter with 1 tablespoon of olive oil over medium heat. Add 4 thinly sliced onions along with a sprig of fresh thyme and a bay leaf. Cook slowly for about 45 minutes, stirring every 10 minutes until the onions are deeply caramelized, taking on a rich, golden hue and sweet aroma.

Once the onions are caramelized, increase the heat to medium-high and pour in ½ cup of white wine, scraping the bottom of the pan to deglaze it. Allow the wine to boil for about 2-3 minutes to evaporate the alcohol. Then, add 6 cups of high-quality beef broth and adjust the heat to bring the mixture back to a boil before reducing it to a simmer for an additional 15 minutes. Season to taste.

While the soup simmers, preheat your broiler to high. Ladle the hot soup into oven-safe bowls, leaving space at the top. Toast thick slices of baguette until golden brown, then place one slice atop each bowl of soup. Generously sprinkle 1 cup of grated Gruyère cheese over the toasted baguette slices, covering them evenly.

Carefully arrange the bowls under the broiler, keeping a close eye on them. Broil for about 3-5 minutes, or until the cheese is melted, bubbly, and lightly golden. Ensure the cheese does not burn; you want it to have an inviting, gooey appearance.

Once the cheese is perfectly melted, remove the bowls from the broiler. If desired, sprinkle a few fresh thyme leaves over the top for a pop of color and extra flavor. Serve immediately while the soup is hot.