Petit Trois (our no. 3 best new restaurant in America) chef Ludo Lefebvre would be the first to tell you: It's not easy to make perfect French onion soup. But fear not—he not only knows how to make the best version we've ever had, but he's also sharing how to make it.
It's all about texture: You want the onions to be cooked through but still crunchy. Lefebvre starts by slicing, not dicing, them and gets the oiled pan ripping hot before adding them in. "Be careful how you cook the onions," warns Lefebvre, who suggests waiting till they turn golden brown before lowering the heat. Leave on low heat for about 10 minutes, and that's it.
Stock is the other focal point of a classic French onion soup. While beef stock is tradition, Lefebvre's not opposed to adding other meats, like veal, as well.
Strain the onions and cook the beef stock and onions together. Fill the French onion soup bowl until it's about 3/4 an inch from the top and add your slice of bread. If there's not enough liquid and bread in the bowl, the cheese will sink, rather than coating the top. Don't do that; don't be that person.
You want the cheese to cover the top. Gruyere is the traditional pick, but Lefebvre works with both emmental and gruyere. This is a man who understands the importance of that moment when the long string of stretchy, gooey melted cheese spans the distance between the bowl and your mouth. He wants that, you want that. Trust him.
Crank up your oven to 375° and put the soup with cheesy top layer in there, rotating occasionally. "You want the bowl to get dirty," Lefebvre says of the cheese dripping down the bowl's side.
Next, take a blowtorch to the cheesy top of the French onion soup to get that layer crispy and bubbling. It's the slightly-charred, yet gooey melted cheese film that rockets this French onion soup off the charts. For home cooks without blowtorches, a broiler can sub in just fine.