Instructions

1. In a large pot over medium heat, melt 4 tablespoons of unsalted butter. Add 4 large sliced onions to the pot and cook, stirring frequently, until the onions are caramelized and golden brown, about 30 to 40 minutes.
2. Once the onions are caramelized, add 2 cloves of minced garlic and cook for an additional 2 minutes until fragrant.
3. Pour in 1 cup of dry white wine to deglaze the pot, scraping up any brown bits from the bottom. Allow the wine to simmer and reduce for about 5 minutes.
4. Add 6 cups of beef broth, 1 teaspoon of dried thyme, 1 bay leaf, and salt and pepper to taste. Bring the mixture to a simmer and let it cook for 20 minutes.
5. While the soup is simmering, preheat the oven to 350°F (175°C).
6. In a bowl, combine 4 cups of shredded Gruyère cheese and 1 cup of shredded Parmesan cheese.
7. Remove the bay leaf from the soup and ladle the hot soup into oven-safe bowls. Place a slice of crusty bread on top of each bowl of soup.
8. Evenly distribute the cheese mixture over the bread in each bowl.
9. Place the bowls on a baking sheet and transfer them to the preheated oven. Bake for 10 to 15 minutes until the cheese is melted and bubbly, and lightly golden.
10. Remove from the oven and let cool slightly before serving.