Caramelize the Onions

1. Melt Butter and Oil: In a large pot, heat the butter and olive oil over medium heat. Once the butter melts, add the sliced onions and stir to coat them in the butter and oil.
2. Cook the Onions: Slowly cook the onions, stirring occasionally, for about 45 minutes to 1 hour, until they become golden-brown and caramelized. Don’t rush this step—caramelizing the onions is the key to developing the deep flavor of the soup.
3. Add Garlic: Once the onions are golden, add the minced garlic and cook for another 1-2 minutes, just until fragrant.

Prepare the Broth

1. Deglaze with Wine: If using wine, pour it into the pot, stirring to scrape up any flavorful bits from the bottom of the pan. Let the wine cook off for about 5 minutes.
2. Add Broth and Herbs: Pour in the beef broth, add the thyme and bay leaf, and season with salt and pepper. Stir everything together.
3. Simmer the Soup: Bring the soup to a boil, then reduce the heat and let it simmer for about 30 minutes. This allows the flavors to meld together beautifully. Taste and adjust seasoning as needed, adding more salt, pepper, or thyme if necessary.

Prepare the Topping

1. Toast the Baguette: While the soup simmers, preheat your oven to 400°F (200°C). Place the baguette slices on a baking sheet and toast them in the oven for about 5-7 minutes, or until they’re golden and crispy.
2. Grate the Cheese: While the bread is toasting, grate your Gruyère and Parmesan cheeses. Set them aside.

Assemble and Serve

1. Prep the Bowls: Ladle the hot soup into oven-safe bowls. Place a slice of toasted baguette on top of each bowl of soup.
2. Top with Cheese: Generously sprinkle the grated Gruyère and Parmesan cheeses over the bread, covering it completely.
3. Broil the Soup: Place the bowls under the broiler for 3-5 minutes, or until the cheese is bubbly and golden brown. Keep an eye on it to ensure it doesn’t burn.
4. Serve: Remove the bowls from the oven carefully (they’ll be hot!) and serve the soup immediately.