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The thought arrived after I had caramelized a variety of onions in butter, then added some homemade chicken stock and let it simmer for a while. Aside from salt, I had put nothing else in the pot. And yet, despite being such a bare-bones version, the soup tasted like one of the best French onion soups I'd ever had.
It made me reflect on all the crappy French onion soup I've been served in my life. Bowls upon bowls of thin burnt-onion tea, the sharp flavor of caramelization gone wrong—something no amount of melted cheese can hope to correct. With nothing more than onions, stock, and salt, it's possible to make one of the most delicious broths in the world. So why are good versions so rare?
The answer lies right there in the question: In the case of such a simple soup, its success or failure comes down to the onions and the broth. Do them right, and you have a masterpiece on your hands. Do them wrong, and it's all lost. Sure, there are things we can do to elevate the soup even further, but they can't stand in for a good base. And the croutons and melted cheese—a requirement for soupe à l'oignon gratinée, and what most Americans think of when we think of French onion soup—should be a bonus, not a crutch.
One of the most common beliefs surrounding French onion soup is that the onions must be cooked to a deep, deep, dark, dark mahogany brown. I'm going to start off by calling BS on that premise. It's not that I think it's wrong to caramelize the onions darkly; I just don't think it's necessary for great results. I also think there's a big risk in going very dark: Unless you're exceedingly careful, it's very easy to introduce unpleasantly bitter flavors to the onion—one of the culprits in so much of the bad French onion soup out there.
In test after test, I found that great French onion soup can be made with more lightly caramelized onions. The deep, sweet flavor that we want arrives long before they turn the color of dark chocolate. And, as I researched other French onion soup recipes, I discovered I wasn't alone in this realization. In fact, some of the people I trust most on this topic have said exactly the same thing.
6 tablespoons (90g) unsalted butter, plus more for bread
3 pounds (1.4kg) yellow or mixed onions, sliced 1/8 inch thick (see note)
Kosher salt and freshly ground black pepper to taste, divided
1/2 cup (120ml) dry sherry (such as Amontillado)
2 quarts (1.8L) homemade or store-bought low-sodium chicken stock
2 sprigs thyme
1 bay leaf
1 teaspoon (5ml) Asian fish sauce (optional)
1 teaspoon (5ml) cider vinegar
8 bowl-size slices rustic bread, toasted until crisp
1 medium clove garlic
1 pound (450g) Gruyère cheese, grated
2 tablespoons 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.