French onion soup is a classic French bistro dish built on slowly caramelized onions, beef or chicken stock, and a crispy Gruyère-topped bread crust. The deep, sweet onion flavor and cheese-laden toast make it a warming, restaurant-quality soup worth the long caramelization time.
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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.
Refrigerate cooled soup (without broiled cheese topping) for up to 3 days. Reheat gently over low heat, then ladle into ovenproof bowls, top with fresh toast and cheese, and broil until golden.
Large stainless steel saucepan or 2 large skillets · Ovenproof serving bowls · Broiler
Initial softening takes about 8 minutes over medium-high heat, then 1 to 2 hours over medium-low heat, stirring frequently, until the onions are very sweet and a rich golden-brown color.
Yes. Prepare the soup through step 3 (after adding fish sauce and cider vinegar), cool, and refrigerate for up to 3 days. Reheat gently before toasting and broiling with cheese in ovenproof bowls.
Swiss, Emmental, or aged white cheddar work well. Avoid pre-shredded cheese, which contains anti-caking agents and won't melt as smoothly.
Fish sauce is optional but enhances umami depth. Omit it if you prefer, though the soup will be slightly less savory.
Yes. Beef stock will give a richer, more traditional French bistro flavor, though chicken stock is equally valid.
