6 large onions, thinly sliced
4 tablespoons unsalted butter
2 garlic cloves, minced
2 bay leaves
2 fresh thyme sprigs
1 cup red wine, preferably a dry variety
3 tablespoons all-purpose flour
8 cups vegetable broth
Salt and freshly ground black pepper, to taste
1 baguette, sliced
1 1/2 cups grated Gruyère cheese
Caramelize the Onions: In a large pot, melt the butter over medium heat. Add the onions, garlic, bay leaves, thyme, and a pinch of salt. Cook, stirring frequently, until the onions are very soft and caramelized, about 25-30 minutes.
Add Wine: Pour in the red wine, bring to a boil, reduce the heat, and simmer until the wine has evaporated and the onions are dry, about 5-10 minutes. Discard the bay leaves and thyme sprigs.
Make Roux: Sprinkle the flour over the onions and stir to coat. Cook for about 10 minutes to cook off the flour taste, stirring frequently.
Add Broth: Add the vegetable broth, bring the soup back to a simmer, and cook for another 10 minutes. Season with salt and pepper to taste.
Toast Bread: While the soup is simmering, toast the slices of baguette in a 350°F (175°C) oven until they are crisp and golden brown.
Broil with Cheese: Preheat the broiler. Ladle the soup into oven-safe bowls. Place a slice or two of toasted baguette on top of each bowl of soup. Sprinkle generously with grated Gruyère cheese.
Broiling: Place the bowls under the broiler and broil until bubbly and golden brown, about 3-5 minutes.
Serve: Carefully remove the bowls from the broiler. Let them cool for a minute, then serve hot.