SERVES: 6 | PREP TIME:
Place a large saucepan over medium-high heat. Add the butter followed by the onions, leeks, salt, and pepper, then cook until the liquid evaporates from the onions and leeks and they just begin to brown, about 8 minutes. Reduce the heat to medium-low and slowly caramelize the onions and leeks, stirring frequently, until they are deep brown in color, about 30 minutes. If the bottom of the bottom of the pan begins to burn, you can add 2 tablespoons of water and stir until the brown bits loosen. Repeat this as many times as needed to prevent burning.
When the onions are deeply caramelized, add the beer and cook until it has evaporated, about 5 minutes. Using butcher’s twine, tie the rosemary and thyme together to form a bouquet. Add the bouquet, broth, salt, and pepper to the onions and leeks. Bring the soup to a boil, then reduce to a simmer and allow to cook for 15 minutes to blend the flavors of the soup.
Preheat the broiler. Place 1 tablespoon of shredded Swiss cheese into the bottom of 6 small (8-ounce) bowls and ladle in an even amount of soup. Top each with a slice of toasted baguette followed by an even amount of the remaining cheese. Transfer the bowls onto a baking pan covered with aluminum foil and broil the top of the soup until the cheese melts and browns, about 4 minutes. Serve immediately. Be sure to take care with the bowls, as they will be very hot from broiling.