Ingredients
1 tablespoon unsalted butter
7 or 8 pounds Spanish onions (about 10 to 15), thinly sliced
Coarse salt
Freshly ground black pepper
6 to 12 slices baguette or any other country-style bread, preferably large enough to cover the width of your serving bowls
1/3 cup sherry
Red- or white-wine vinegar (optional)
Red wine (optional)
1/2 to 3/4 pound Gruyere or Emmenthaler cheese, grated
Preparation
Place a 7 1/2-quart enameled cast-iron pot over medium heat; melt butter. Add onions and sprinkle with 2 teaspoons salt; cover, and cook until onions have heated through and started to steam. Uncover and reduce heat to low; cook, stirring occasionally, until amber and cooked down, about 3 hours. Season with pepper.
Meanwhile, preheat oven to 200 degrees.
Place bread slices on a baking sheet and transfer to oven. Let dry completely, 20 to 30 minutes.
Add 6 cups water to pot with onions; increase heat to high and bring to a simmer. Immediately reduce heat to low. Add sherry; season with salt and pepper. Add a splash of vinegar, if desired, to cut sweetness. For more depth of flavor add a splash of red wine, if desired.
Preheat broiler. Divide soup evenly among 4 to 6 ovenproof bowls; top with toasted bread. Divide cheese evenly among bowls. Transfer bowls to broiler; broil until cheese is melted and browned, 3 to 5 minutes. Serve immediately.
