The Unconventional Ingredient That Makes Serious Eats’ French Onion Soup So Good

Serious Eats is known for writing solid recipes that are rigorously tested. As their name implies, they take recipe testing seriously. So when picking out recipes to include in our French onion soup showdown, I felt confident that their version of the classic would be a strong contender.

This recipe calls for a handful of simple upgrades — like using a combination of onion varieties versus just one — but nothing too obscure. And it includes a rather unexpected ingredient that I had never seen used in French onion soup before, so I was excited to give it a try. Here’s what happened when I took to the kitchen.

Get the recipe: Serious Eats’ French Onion Soup (Soupe à l’Oignon Gratinée)

Serious Eats’ recipe starts by having you cook several different types of sliced onions — red, yellow, sweet, and shallots — over medium-high heat with butter until softened. You’ll then reduce the heat to low and cook until the onions are sweet and a “rich-golden color.” Serious Eats notes that this can take anywhere between one to two hours, and I found that it took just over an hour to achieve a perfect golden color. If the onions start to burn, add a tablespoon of water and scrape up any brown bits.

You’ll then add a splash of sherry, cook until the “alcohol smell is mostly gone,” then add low-sodium chicken stock, thyme, and a bay leaf. As the soup cooks, you’ll butter slices of toasted bread and rub them with a garlic clove.

Lastly, you’ll add both apple cider vinegar and fish sauce, remove the herbs, ladle half the soup into four oven-safe bowls and top each with a piece of the buttered toast and some shredded Gruyère. You’ll ladle the remaining soup over top, top with another layer of toast and cheese, then broil until the cheese is melted and browned. Garnish with minced fresh chives.

This soup was everything I wanted French onion soup to be and more. It was rich, ultra-savory, and seriously comforting. The two layers of melted Gruyère and toasted bread gave it that classic heartiness I associate with a good French onion soup, and the chives added a pleasant pop of freshness that helped cut the richness of the broth.

The combination of yellow onions, sweet onions, red onions, and shallots made the soup taste more complex and onion-y — in a good way! It was a small detail that made the broth taste more complex and exciting, and it was well worth the effort of picking out a few different onion varieties at the market.

Another thing I loved about this recipe was its use of chicken stock. I was nervous it would make the soup pale and flavorless, but it turned out to be delicious. I didn’t miss the beef broth at all, and to be honest I probably wouldn’t have even known it wasn’t made with beef broth. It still had a deep richness, only a bit lighter.

But the one thing that made this soup truly stand out was the fish sauce. It didn’t make the broth taste fishy; rather, it amped-up the savoriness and took it to a whole new level. I will be adding fish sauce to every French onion soup I make from here on out.

1. Don’t skip the fish sauce: Although it might sound weird, fish sauce is actually the perfect ingredient to amp-up the savoriness of French onion soup. The recipe lists it as an “optional” addition, but don’t even think about skipping it! It’s one of the main reasons why I loved this soup so much.

2. Use a combination of several different types of onions, not just yellow: Serious Eats suggests using a combination of several different varieties of onions to make the soup, and it really did make a difference. Red, yellow, and sweet onions, along with a few shallots, make an excellent combination.

3. Feel free to skip rubbing the bread with garlic : This step was a bit tedious, and once the bread was in the soup you couldn’t even taste it. My advice? Toast the bread and ditch the garlic.

4. Go heavy on the chives: Although it might sound like a useless garnish, the chives added a wonderful freshness to the soup and a pleasant pop of onion flavor.

Rating: 9/10

Have you ever made Serious Eats’ French onion soup? Tell us what you thought!