French Onion Soup

Adjust the oven rack to the lower-middle position and preheat the oven to 400 degrees F. Spray the inside of a 7 quart Dutch oven with a nonstick cooking spray.

Place the butter in the Dutch oven and add the sliced onions and 1 teaspoon salt. Cook in the preheated oven, with the cover on the pot, for 1 hour (the onions will be moist and slightly reduced in volume). Remove the pot from the oven and stir the onions, scraping the bottom and sides of the pot. Return the pot to the oven with the lid slightly ajar and continue to cook until the onions are very soft and golden brown, 1½ hours longer, stirring the onions and scraping bottom and sides of pot after 1 hour.

Remove the pot from the oven and place over medium-high heat. 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. Continue to cook, stirring frequently, until the pot bottom is coated with a dark crust, roughly 6 to 8 more minutes, adjusting the heat as necessary. (Scrape any fond that collects on the spoon back into onions.)

Stir in ¼ cup of chicken broth, scraping the pot bottom to loosen crust, and cook until liquid has evaporated and pot bottom has formed another dark crust, 6 to 8 minutes. Repeat process of deglazing with 1/4 cup increments of chicken broth for 3 more times, until onions are very dark brown. Then stir in the sherry and cook, stirring frequently, until the sherry evaporates, about 5 minutes. Stir in the remaining broths, thyme, bay leaves, 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. Then reduce the heat to low, cover, and simmer for 30 minutes.

Remove and discard thyme and bay leaves, then season with salt and pepper.

Toast the slices of French bread in the oven for about 5 minutes. Transfer the soup to smaller broiler-safe soup bowls; top with the bread and shredded gruyere. Place the bowls of soup under the broiler until the cheese is brown and bubbly.