- 4 tablespoons unsalted butter
- 3 pounds yellow onions, thinly sliced
- 1 1/2 teaspoons salt
- 1/2 teaspoon freshly ground black pepper
- 1/3 cup cognac
- 8 cups beef stock or broth
- 4 sprigs fresh thyme, tied into a bundle with kitchen string
- 1/2 loaf French bread, cut into 1/2-inch thick slices
- 1 pound Gruyere cheese, coarsely grated
- 2 egg yolks (optional)
- 1/2 cup Port wine (optional)
- Finely chopped parsley, garnish
- In a Dutch oven or other large, heavy pot, melt the butter over medium-high heat. Add the onions, salt and pepper, and cook, stirring, until golden brown, 15 to 18 minutes. Remove the pan from the heat and carefully add the cognac. Return the pan to the heat and cook until the alcohol has evaporated. Be careful as the cognac may ignite.
- Add the beef stock and thyme sprigs and bring to a boil. Reduce the heat to a simmer and cook the soup for 45 minutes.
- While the soup is simmering, toast the bread slices until light golden brown. Remove from the oven.
- Preheat the broiler.
- When the soup is ready, divide 1/2 of the toasted bread slices between 6 individual ovenproof serving bowls or crocks and top with 1/2 of the grated cheese. Ladle some of the soup among the bowls and top with the remaining toasts. Ladle the remaining soup among the bowls and top with the remaining cheese. Place the bowls on a baking sheet and place under the broiler until the cheese is melted, golden brown and bubbly, about 5 minutes. Remove from the oven.
- Optional topping: In a small bowl combine the egg yolks and Port and whisk to thoroughly combine. Pour some of the mixture evenly among the soup bowls, stirring in around the edges so that it is incorporated into the soup. (The heat of the soup will cook the egg yolk and this will thicken and enrich the soup.)
- Garnish the top with chopped parsley and serve hot.