Six-Onion Soup with Parmesan Croutes
To prepare soup: Heat 3 tablespoons oil in a large stockpot over medium heat. Add leeks and cook, stirring occasionally, until softened but not browned, 8 to 10 minutes. Add shallots, scallions and garlic; cook, stirring, for 1 minute. Add sweet onions, red onions, salt, pepper and the remaining 3 tablespoons oil; stir to combine. Cook, stirring occasionally, until the onions are reduced by half and very juicy, about 45 minutes.Advertisement
Add parsley and thyme; cook, stirring occasionally, until the mixture is deeply browned, 35 to 45 minutes more.
Increase heat to high and add cognac (or sherry). Cook, scraping up any browned bits, for 1 minute. Add broth, cover and bring to a boil. Reduce heat, partially cover and simmer for 15 minutes.
To prepare croutes: Just before serving, position a rack in upper third of oven; preheat broiler on low. Place baguette slices on a baking sheet and brush one side with oil. Broil until golden brown, 4 to 6 minutes. Turn the slices over and top each with a generous 1 tablespoon Parmesan. Broil until the cheese is melted, 4 to 6 minutes more. Serve the soup topped with croutes.
To make ahead: Refrigerate soup for up to 3 days or freeze for up to 3 months.