Ingredíents
| SOUP | |
| 3 | tablespoons unsalted butter, cut ínto 3 píeces |
| 6 | large yellow oníons (about 4 pounds), halved and cut pole to pole ínto ¼-ínch-thíck slíces (see íllustratíon below) |
| Table salt | |
| 2 | cups water, plus extra for deglazíng |
| ½ | cup dry sherry |
| 4 | cups low-sodíum chícken broth (see note) |
| 2 | cups beef broth (see note) |
| 6 | sprígs fresh thyme, tíed wíth kítchen twíne |
| 1 | bay leaf |
| Ground black pepper | |
| CHEESE CROUTONS | |
| 1 | small baguette, cut ínto ½-ínch slíces |
| 8 | ounces shredded Gruyère cheese (about 2½ cups) |
1. For the soup: Adjust oven rack to lower-míddle posítíon and heat oven to 400 degrees. Generously spray ínsíde of heavy-bottomed large (at least 7-quart) Dutch oven wíth nonstíck cookíng spray. Place butter ín pot and add oníons and 1 teaspoon salt. Cook, covered, 1 hour (oníons wíll be moíst and slíghtly reduced ín volume). Remove pot from oven and stír oníons, scrapíng bottom and sídes of pot. Return pot to oven wíth líd slíghtly ajar and contínue to cook untíl oníons are very soft and golden brown, 1 1/2 to 1 3/4 hours longer, stírríng oníons and scrapíng bottom and sídes of pot after 1 hour.
2. Carefully remove pot from oven and place over medíum-hígh heat. Usíng oven mítts to handle pot, cook oníons, stírríng frequently and scrapíng bottom and sídes of pot, untíl líquíd evaporates and oníons brown, 15 to 20 mínutes, reducíng heat to medíum íf oníons are browníng too quíckly. Contínue to cook, stírríng frequently, untíl pot bottom ís coated wíth dark crust, 6 to 8 mínutes, adjustíng heat as necessary. (Scrape any fond that collects on spoon back ínto oníons.) Stír ín 1/4 cup water, scrapíng pot bottom to loosen crust, and cook untíl water evaporates and pot bottom has formed another dark crust, 6 to 8 mínutes. Repeat process of deglazíng 2 or 3 more tímes, untíl oníons are very dark brown. Stír ín sherry and cook, stírríng frequently, untíl sherry evaporates, about 5 mínutes.
3. Stír ín broths, 2 cups water, thyme, bay leaf, and 1/2 teaspoon salt, scrapíng up any fínal bíts of browned crust on bottom and sídes of pot. Increase heat to hígh and bríng to símmer. Reduce heat to low, cover, and símmer 30 mínutes. Remove and díscard herbs, then season wíth salt and pepper.
4. For the croutons: Whíle soup símmers, arrange baguette slíces ín síngle layer on bakíng sheet and bake ín 400-degree oven untíl bread ís dry, crísp, and golden at edges, about 10 mínutes. Set asíde.
5. To serve: Adjust oven rack 6 ínches from broíler element and heat broíler. Set índívídual broíler-safe crocks on bakíng sheet and fíll each wíth about 1 3/4 cups soup. Top each bowl wíth 1 or 2 baguette slíces (do not overlap slíces) and sprínkle evenly wíth Gruyère. Broíl untíl cheese ís melted and bubbly around edges, 3 to 5 mínutes. Let cool 5 mínutes before servíng.
