In a large, heavy-bottomed pot, combine the unsalted butter and extra virgin olive oil over medium heat, allowing it to melt and fuse.
Carefully add the thinly sliced onions and the granulated sugar to the pot. Sauté the mixture, stirring frequently, for approximately 30 to 40 minutes until the onions are caramelized.
Once the onions are caramelized, stir in the minced garlic and cook for an additional 2 minutes.
Gradually pour in both the beef and vegetable broth, then add the fresh thyme, bay leaf, and season with salt and black pepper. Bring to a gentle simmer and cook for about 20 minutes.
While the soup simmers, preheat your oven to the broil setting.
Remove the bay leaf from the soup and ladle it into oven-safe bowls.
Place a slice of toasted baguette atop each bowl of soup and layer with Gruyère and Parmesan cheese.
Place the bowls under the broiler for about 3 to 5 minutes until the cheese is bubbly and golden brown.
Carefully take the bowls out of the oven and allow them to cool slightly before serving.
Notes
Serve in rustic, warm bowls and garnish with fresh thyme leaves. Drizzle with truffle oil for an elegant touch.