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 one teaspoon salt. (The salt helps draw out the excess moisture from the onions.) Cook, covered, one 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, checking the pan at about 40 minutes.
Stir and scrape the bottom and sides of the pan, then check every 15 minutes or so until they are soft and brown and nearly dry on the bottom. Remove pan if it appears to be in any danger of burning.
Note: The above is a deviation from Cook’s Illustrated – the first time I made this, following their directions my unions were burnt and dry the second time I checked them.
Carefully remove pot from 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 liquid evaporates completely and onions turn to a dark brown, 15 to 20 minutes, reducing heat to medium if onions are browning too quickly. Continue to cook, stirring frequently, until pot bottom is coated with dark crust, 6 to 8 minutes, adjusting heat as necessary. (Scrape any gooey fond that collects on spoon back into onions.)
Stir in 1/4 cup water, scraping pot bottom and sides to loosen crust, and cook until water evaporates and pot bottom has formed another dark crust, 6 to 8 minutes. If your onions are not a dark brown, repeat this step. (Look for the color in the bottom photo.)
Stir in sherry and cook, stirring frequently, until sherry evaporates, about 5 minutes.
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 gently 30 minutes. Remove and discard herbs, then season with salt and pepper.
For the croutons & serving:
While soup simmers, arrange baguette slices in single layer on baking sheet and bake in 400-degree oven until bread is dry, crisp, and golden at edges, about 10 minutes. Set aside.
Adjust oven rack 6 inches from broiler element and heat broiler. Set individual broiler-safe crocks on baking sheet and fill each with about 1 3/4 cups soup, leaving enough room at the top for the croutons. Top each bowl with 1 or 2 baguette slices (do not overlap slices) and sprinkle evenly with Gruyère. Broil until cheese is melted and bubbly around edges, 3 to 5 minutes. Let cool 5 minutes before serving.
Per Serving: 623 Calories; 41g Fat (60.8% calories from fat); 45g Protein; 15g Carbohydrate; 4g Dietary Fiber; 134mg Cholesterol; 1145mg Sodium. Exchanges: 0 Grain(Starch); 6 Lean Meat; 1 1/2 Vegetable; 5 Fat.
Keywords: Bargain Meal of the Week, Beef Stock, Bread, Caramelized Onion, Cheese, Chicken Stock, Cook's Illustrated, Freezes Well, French, Kitchen Slicer, leftover bread, Leftovers, onions, Soup