All hail the French onion soup at Les Zygomates

The culinary classic never falls out of favor at this established eatery.

The French onion soup at Les Zygomates is so popular, customers even order when temperatures are soaring in August. John Tlumacki/Globe staff

Brazilian-born chef Guaracyara Pimenta is surprised by how many customers order French onion soup at Les Zygomates. The kitchen prepares 15 to 20 bowls a night, and more during the day. “People order it all year,” says the chef. “Even in the heat of summer in August.”

He begins with mostly Spanish onions, which he cooks with garlic, a little sugar, and a bouquet garni of thyme, bay leaf, and peppercorns (it comes out later). He lets the onions soften and caramelize to a deep golden color, then adds sherry and brandy and reduces them in the pan. The broth is the kitchen’s stock, made with beef and veal bones, and he adds only enough to the pot so that the onions dominate the soup bowl. As orders come in, the chef ladles hot soup into individual heatproof white casseroles and adds toasted rounds of bread, then tops the bread with a thin slice of Swiss cheese, which acts as a raft for a heavy dusting of grated gruyere (the Swiss keeps the gruyere from sinking). He slides the dish under a hot salamander (broiler), which makes the traditional golden crust, sprinkles the top with chives, and sends the bowl out piping hot.

When you dip your spoon in, the cheese forms beautiful melted strands as you pull it away from the bowl. It’s fragrant, sweet, deeply oniony, and comforting. “It’s a classic, and they love it so much,” Pimenta says of his customers.