I got this from the Serious Eats website from following a link on his "Popovers" page, which I copied onto MY popovers page. He says he used a batch of this recipe to make a gravy for the popovers; he reduced it to half, added a splash of soy sauce, and thickened it with a cornstarch slurry. Very interesting.
Ingredients
6 tablespoons unsalted butter, plus more for toasts
3 pounds yellow or mixed onions, sliced 1/8 inch thick
Kosher salt and freshly ground black pepper
1/2 cup dry sherry (such as Amontillado)
2 quarts homemade or store-bought low-sodium chicken stock
2 sprigs thyme
1 bay leaf
1 teaspoon Asian fish sauce (optional)
1 teaspoon cider vinegar
8 bowl-size slices rustic bread, toasted until crisp
1 medium clove garlic
1 pound Gruyere cheese, grated
Freshly minced chives, for garnish
Instructions
1. In a large stainless steel or cast iron saucepan, or two large skillets, melt butter over medium-high heat until foaming. Add onions and cook, stirring occasionally, until softened, about 8 minutes. Lower heat to medium-low and cook, stirring frequently, until onions are very sweet and a rich golden brown color, 1 to 2 hours. If browned onion juices on the bottom of the pot threaten to burn, add 1 tablespoon of water, scrape up the browned bits, and continue cooking. Season with salt and pepper.
2. Add sherry and bring to a simmer, scraping up any browned bits (if using 2 skillets, divide sherry between them, then scrape onions and liquid from both pans into a pot or Dutch oven to continue). Cook until alcohol smell is mostly gone, about 3 minutes. Add stock, thyme, and bay leaf, raise heat to medium-high and bring to a simmer. Lower heat and simmer for 20 minutes.
3. Add fish sauce, if using, and cider vinegar and season with salt and pepper. Discard thyme stems and bay leaf.
4. Preheat broiler and position oven rack to top position. Butter toasts and rub with garlic clove until fragrant. Spoon a small amount of broth into the bottoms of 4 oven-proof serving bowls, then top with half the toasts. Sprinkle some grated cheese on top of toasts, then spoon more soup and onions on top, nearly filling the bowls. Set the remaining 4 toasts in each bowl, pushing to nearly submerge them. Top with remaining grated cheese and set bowls on a rimmed baking sheet. Broil until cheese is melted and browned in spots. Garnish with chives and serve.