The Pantry Sauce Ludo Lefebvre Uses For The Best French Onion Soup

Comforting and cozy, the aroma of a bowl of French onion soup is the very definition of inviting. Next time you're whipping up a batch, consider boosting its meaty flavor and fragrance further with a tip from chef Ludo Lefebvre: Add a dash of Worcestershire sauce at the very end of the cooking time to give your soup oodles of umami character. Worcestershire sauce is a savory condiment made of vinegar, molasses, anchovies, tamarind, and garlic. It has a distinctive, piquant smell and imbues dishes with instant depth because some of the ingredients, such as anchovies, are high in an amino acid called glutamate, which lends foods an umami quality. 

In a 2022 interview with Food & Wine, Lefebvre explains that adding a splash of Worcestershire to a simmering pan of onion soup "really brings back more flavors of the beef." We reckon he means that the sauce accentuates the beefy flavor of the meaty stock that serves as the base of this classic peasant-style meal, thereby magnifying its intensity. While preparing the dish, Lefebvre takes care to stir the sauce into the soup after it has simmered gently for 30 minutes rather than incorporating it at the very beginning. At this point in the process, the onions are tender and silky, and the broth has condensed down so the sauce almost awakens and rounds off its flavor. This way, the Worcestershire fills in any gaps with its tart, sweet-and-sour personality.

Due to its concentrated nature, Worcestershire sauce is used sparingly, much like soy or fish sauce. It also has a thinner consistency, so it can be seamlessly mixed into dull stews and casseroles to enliven their flavor. The key to incorporating Worcestershire sauce into French onion soup is to start small. First add a little taste for seasoning, and then some more if required. Just a few dashes will be sufficient for the umami quality of the fermented sauce to underscore the savoriness of the meaty stock. 

While adding Worcestershire sauce to any soup will boost its flavor, the secret to making the best French onion soup is time. You must give your onions plenty of time to cook down and develop a golden color as the natural sugars inside the flesh caramelize. This produces that deep oniony flavor you're looking for. Indeed, a classic French onion soup is a pared-back combination of beef stock, seasonings, and caramelized onions, so each ingredient has to be at its very best. 

Once ladled into bowls, the soup is gratinéed (broiled with chunks of toasty bread topped with cheese) just before serving to help the toasted slices retain their crispy texture for as long as possible. As the soup sits, the bread soaks up the meaty broth, which has sweetened slightly with the flavor of the slow-cooked onions, thereby providing heaps of satisfying texture.