Ingredients for servings
90 g unsalted butter, plus more for toasts
3 pounds yellow or mixed onions, french cut see note
kosher salt and freshly ground black pepper
120 ml dry sherry such as Amontillado
2 pounds homemade or store-bought low-sodium chicken stock
2 sprigs thyme
1 bay leaf
5 ml Asian fish sauce optional
5 ml cider vinegar
8 bowl-size slices rustic bread, toasted until crisp
1 medium clove garlic
450 g Gruyère cheese, grated
freshly minced chives, for garnish
In a large stainless steel saucepan, or in 2 large stainless steel or cast iron 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 bottom of pot threaten to burn, add 1 tablespoon (15ml) water, scrape up browned bits, and continue cooking. Season with salt and pepper.
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.
Add fish sauce, if using, and cider vinegar. Season with salt and pepper. Discard thyme sprigs and bay leaf.
Preheat broiler and adjust 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 ovenproof 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 bowls. Set remaining 4 toasts in bowls, 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.
Caramelizing the onions slowly in butter until they’re rich golden-brown (but not so dark as to taste bitter) produces the sweetest, most flavorful results.
The homemade or store-bought chicken stock used here is typically better-quality than the store-bought beef stock many recipes call for.
Asian fish sauce, cider vinegar, and sherry add depth and complexity to the broth.
Yellow onions are your best all-purpose bet for this soup, but a mix of yellow, sweet (e.g., Vidalia), and red onions, as well as shallots, produces an even more complex flavor.