The Unconventional Ingredient Lidia Bastianich Uses To Elevate Onion Soup

Of all the hearty, comforting soups out there, French onion just might be one of the most delicious. Consisting of sliced onions that are caramelized in butter until tender and deeply sweet, then simmered in beef stock and topped with a raft of toasted bread weighted down with plenty of melted Gruyère cheese, this Gallic classic has stood the test of time. 

According to VICE, French onion soup has been around since as early as the 14th century, when an early crouton-less version appeared in Taillevent's cookbook "Viander," but it's popularity truly began to take off in 1930s and '40s Paris thanks to restaurants like La Poule au Pot and Au Pied de Cochon. That's when, the outlet cites, various working-class restaurants surrounding the city's hulking Les Halles market decided to add the tender-underneath, crispy-on-top slice of bread snowed in under a mountain of gratinéed cheese. The rest, as they say, is history.

Over the decades, the soup has remained true to its roots, with recipes tending to stick quite closely to the original. If it ain't broke, don't fix it, right? But that doesn't mean that some minor variations aren't more than welcome, such as the addition of smoky poblanos or a bread swap. And if there's anyone we trust to adapt French onion soup into something even more delicious, it's Lidia Bastianich, Italian cooking doyenne and host of numerous PBS cooking shows including "Lidia's Italy" (via PBS).

A handful of mushrooms brings depth to this rich soup

A dynamic duo of the food world, mushrooms and onions appear together in so many of the dishes we love, from mushroom risotto to stuffed mushrooms. The sweetness of onions seems to perfectly complement the rustic earthiness of mushrooms, a flavor combination that Italian cooking expert Lidia Bastianich calls upon in her adaptation of French onion soup. In it, she sautées 10 ounces of sliced white mushrooms with the onions, lending depth of flavor to the rich soup.

As Bastianich recalls in the recipe, developing the soup was a classic case of using up refrigerator odds 'n' ends. "When we were testing this recipe in my kitchen," she writes, "I had a box of mushrooms in my refrigerator and wanted to use them up. When we sat down to lunch that afternoon, and Tanya tasted the soup, she said, 'I don't know why anyone hasn't thought of this before! Onion soup should always have mushrooms in it!'"

Further Italian touches to the soup include using slices of Italian bread as the crouton base, and fontina — that mild, nutty cheese that melts exceedingly well — in place of the Gruyère. All in all, this Italian take on onion soup sounds like something we want to get to simmering, ASAP.