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1
For the soup:.
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Adjust the oven rack to the lower-middle position and heat the oven to 400 degrees.
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3
Generously spray the inside of a heavy-bottomed large (at least 7-quart) Dutch oven with a nonstick cooking spray.
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4
Place the butter in the pot and add the onions and 1 teaspoon salt.
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5
Cook, covered, for 1 hour.
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6
Remove the pot from the oven and stir the onions, scraping the bottom and sides of the pot.
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7
Return the pot to the oven with the lid slightly ajar and continue to cook until the onions are very soft and golden brown, 1 1/2 to 1 3/4 hours longer, stirring the onions and scraping bottom and sides of pot after 1 hour.
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8
Carefully remove pot from oven and place over medium-high heat.
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9
Using oven mitts to handle pot, cook onions, stirring frequently and scraping bottom and sides of pot, until the liquid evaporates and the onions brown, 15 to 20 minutes, reducing the heat to medium if the onions are browning too quickly.
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10
Continue to cook, stirring frequently, until the pot bottom is coated with a dark crust, roughly 6 to 8 minutes, adjusting the heat as necessary.
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11
Scrape any fond that collects on spoon back into onions.
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12
Stir in 1/4 cup water, scraping the pot bottom to loosen crust, and cook until water evaporates and pot bottom has formed another dark crust, 6 to 8 minutes.
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13
Repeat process of deglazing 2 or 3 more times, until onions are very dark brown.
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14
Stir in the sherry and cook, stirring frequently, until the sherry evaporates, about 5 minutes.
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15
Stir in the broths, 2 cups of water, thyme, bay leaf, and 1/2 teaspoon salt, scraping up any final bits of browned crust on bottom and sides of pot.
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16
Increase heat to high and bring to simmer.
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17
Reduce the heat to low, cover, and simmer for 30 minutes.
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18
Remove and discard herbs, then season with salt and pepper.
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19
For the croutons:.
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20
While the soup simmers, arrange the baguette slices in single layer on baking sheet and bake in a 400-degree oven until the bread is dry, crisp, and golden at edges, about 10 minutes.
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21
Set aside.
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22
To serve:.
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23
Adjust oven rack 6 inches from broiler element and heat broiler.
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24
Set individual broiler-safe crocks on baking sheet and fill each with about 1 3/4 cups soup.
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25
Top each bowl with 1 or 2 baguette slices (do not overlap slices) and sprinkle evenly with Gruyere.
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26
Broil until cheese is melted and bubbly around edges, 3 to 5 minutes.
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27
Let cool 5 minutes before serving.
