3 tablespoons unsalted butter, cut into 3 pieces
6 large yellow onions (about 4 pounds), halved and cut pole to pole into 1/4-inch-thick slices
Table salt
2 cups water, plus extra for deglazing
1/2 cup cooking sherry
4 cups low-sodium chicken broth
2 cups beef broth
1 teaspoon thyme
1 bay leaf
Ground black pepper
1 baguette, cut into 1/2-inch slices
8 ounces shredded Gruyère cheese (about 2 1/2 cups)
1. For the soup: Adjust oven rack to lower-middle position and heat oven to 400 degrees. Generously spray inside of heavy-bottomed large (at least 7-quart) Dutch oven with nonstick cooking spray. Place butter in pot and add onions and 1 teaspoon salt. Cook, covered, 1 hour (onions will be moist and slightly reduced in volume). Remove pot from oven and stir onions, scraping bottom and sides of pot. Return pot to oven with lid slightly ajar and continue to cook until onions are very soft and golden brown, 1 1/2 to 1 3/4 hours longer, stirring onions and scraping bottom and sides of pot after 1 hour.
2. Carefully remove pot from oven and place over medium-high heat. Using oven mitts to handle pot, stir in 1/4 cup water, scraping pot bottom to loosen crust, and cook until water evaporates and pot bottom has formed another dark crust, 6 to 8 minutes. Repeat process of deglazing 2 or 3 more times, until onions are very dark brown. Stir in sherry and cook, stirring frequently, until sherry evaporates, about 5 minutes.
3. Stir in broths, 2 cups water, thyme, bay leaf, and 1/2 teaspoon salt, scraping up any final bits of browned crust on bottom and sides of pot. Increase heat to high and bring to simmer. Reduce heat to low, cover, and simmer 30 minutes. Remove and discard bay leaf, then season to taste with salt and pepper.
4. For the croutons: Arrange baguette slices in single layer on baking sheet. Sprinkle evenly with Gruyère. Broil until cheese is melted and bubbly around edges, 3 to 5 minutes.
5. Ladle the soup into bowls and add croutons on top. Serve.
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