Cut the onions into quarter sections and place on a smoking rack. Smoke for 2 to 4 hours at 200 degrees until the onions are soft and beginning to open up. Best if you do this the day before, the smoke flavor will intensify. Then slice the onions thin.

In a large pot, melt the butter with olive oil over medium heat. Add sliced onions and cook, stirring occasionally, until caramelized and golden brown, about 30-40 minutes. Stir in minced garlic, thyme, and the bay leaf. Cook for another minute to release the flavors. Pour in beef broth, chicken broth, and wine (if using). Bring the mixture to a boil, then reduce heat and let it simmer for 15-20 minutes. Season with salt and pepper to taste. Preheat your broiler. Ladle the soup into oven-safe bowls. Top each with a slice of toasted bread and a generous sprinkle of shredded Gruyère cheese.

Broil for 2-3 minutes until the cheese is melted and bubbly. Carefully remove the bowls from the oven, let cool slightly, and serve.