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How to Make French Onion Soup in a Slow Cooker

How to Make French Onion Soup in a Slow Cooker

We almost wish the weather turns cold enough for this great soup

The hardest part of this whole recipe is cutting the onions.

It’s irresistible. An incomparable heartwarming bowl of caramelized onions and rich beef stock topped with toasted French bread and melted Comté cheese makes you wish for cold weather. For all its goodness, it’s really not all that hard to make—except for those caramelized onions. They’re the heart of the dish.  But they require great patience to achieve greatness. At least 45 minutes despite recipes that trim the time they take.  Lo and behold, along comes this recipe. It’s from the Editor in Chief of Kitchn, Faith Durand and it’s amazing. The most effort required is in cutting the onions. And then basically doing nothing at all for the next 12 hours. The dark golden-brown onions perfected, the next step is to add the beef broth and balsamic vinegar and wait another 6 to 8 hours!  20 hours may be an eternity to wait. But you’ll be richly rewarded for your patience.

Nicolas Appert, whose recipe for Onion Soup was dedicated to the Duke of Lorraine, father of Queen Marie of France

While you wait, read the story of how French Onion Soup came into being.

Louis XV. Does this man look like he ever set foot in a kitchen?

Onion soup goes back to at least ancient Rome and likely a lot earlier.  Onions are easy to grow and they’ve always been inexpensive so everyone from the poor to the nobility ate them. The modern version of French Onion Soup dates from the 18th century and the reign of Louis XV. Story number one involves Nicolas Appert, the food scientist who invented canning. Reportedly, a young Appert cooked at a hotel called La Pomme d’Or in Châlons-en-Champagne. The Duke of Lorraine, ex-King of Poland, stopped at the hotel en route to Versailles to visit his daughter Queen Marie, the wife of Louis XV.  Appert prepared an onion soup. The duke was so taken with the soup that he insisted on watching it being prepared to take the recipe home.  In 1831, Appert published a cookbook dedicating the onion soup recipe to the duke. A second tale had Louis XV himself invent the soup.  This legend has the king returning from hunting to find only butter, onions, and champagne.  He and his great aunt improvised and cooked them all together in a soup that the Royal Court was mad for.  Seriously? A King in the kitchen? Highly doubtful. Especially one with a reputation of being a womanizer and a notoriously bad monarch.

So when did French Onion Soup take on its signature Bread and Onion Topping?

Swells swarmed Les Halles for its Onion Soup.

Les Halles, the central marketplace in Paris was founded in 1135.  It was so popular, it encroached on a cemetery.  Not even a high wall could keep out the stench. Finally, the rotting bones were removed to the Catacombs in the 19th Century and Les Halles came into its own. The entire market was ringed with inexpensive bistros to feed the workers. These bistros came up with soaking bread in the soup and broiling cheese on top of it.  Since much of the work in the market was done either late in the night or early in the morning, take your pick, Parisian party-goers, and bon vivants ended their nights at these 24-hour restaurants.  Sitting side by side, the very rich and the very poor came together over bowls of onion soup.  It didn’t hurt that the soup developed a reputation as being a great cure for a hangover. There’s not a lot of science to back that up. But onions are one of the healthiest of all vegetables and salt can help replace lost electrolytes. Here is today’s recipe. And after it some other winter soups we love.

Slow Cooker French Onion Soup

November 5, 2020
: 6 to 8
: 20 min
: 20 hr
: If you can slice an onion, you've got this.

An incomparable heartwarming bowl of caramelized onions and rich beef stock topped with toasted French bread and melted Comté cheese.


  • 3 pounds yellow onions, peeled, sliced, and cut into quarter-moons
  • 2 tablespoons unsalted butter, melted
  • 2 tablespoons olive oil
  • 2 teaspoons kosher salt, plus more as needed
  • Freshly ground black pepper
  • 10 cups reduced-sodium beef broth
  • 2 tablespoons balsamic vinegar
  • 3 tablespoons brandy (optional)
  • To Serve
  • 4 to 6 toasted baguette slices per bowl
  • 1/3 cup grated Comté or Gruyère cheese per bowl (1 1/3 to 2 cups total)
  • Step 1 Season the onions. Place the onions in a 5-quart or larger slow cooker. Stir in the butter, oil, salt, and a generous amount of pepper.
  • Step 2 Cook on LOW for 12 hours. Cover and cook on the LOW setting overnight until the onions should be dark golden-brown and soft, 12 hours or overnight.
  • Step 3 Add the broth and vinegar. Stir in the broth and vinegar.
  • Step 4 Cook for LOW 6 to 8 hours. Cover and continue cooking on the LOW setting for 6 to 8 hours. This is flexible
  • Step 5 as long as your slow cooker holds moisture well (wrap a towel over the lid if quite a lot of steam escapes), you can cook the soup for hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if needed, and stir in the brandy if using.
  • Step 6 Portion the soup into oven-safe bowls. Arrange a rack in the upper third of the oven and heat to 350°F. Ladle the soup and onions into oven-safe soup bowls and place the bowls on a rimmed baking sheet.
  • Step 7 Top with toast and shredded cheese. Top each bowl with a slice of toast and a generous quantity of shredded Gruyère cheese, about 1/3 cup per bowl.
  • Step 8 Bake for 20 to 30 minutes. Bake until the cheese is completely melted, 20 to 30 minutes.
  • Step 9 Broil for 2 to 3 minutes. Turn the oven to broil. Broil until the cheese is bubbling and browned 2 to 3 minutes. Remove from the oven and let cool for a few minutes.

A Tale of Three Soups: Jim’s Hurricane Survival Soup, Alice Waters’ Spicy Cauliflower Soup and Cousin Bar’s Pear and Parsnip Soup

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6 thoughts on “How to Make French Onion Soup in a Slow Cooker”

  • Dear Monte,

    This is simmering overnight as I write, snuggled under my down duvet. Brrr! We have a(nother) BIG snowstorm blowing in.
    I can’t wait to eat this tomorrow with my dh and my son. I love French onion soup. My go-to has always been Jacques Pepin’s but this sounds way too easy and therefore fun! I’m sure it’ll be outstanding.

    I made Joanne Chang’s banana bread this past week and am going to make my absolute fave; Bills carrot cake on Sunday. My S-I-L comes home Monday from 3 weeks camp up north and we are all making him his favorites for his arrival.

    We (Canadians) are all sitting on the edge of our seats as you all await the outcome of the election.

    In the meantime, I’m making all your recipes from the recent past: cheeseburger chopped steak with tomatoes on the side, Loco Moco and Melissa Clark’s B&W cookies.


    Thanks, Monte! Stay safe!


    • Always so good to hear from you Kate! And you can’t imagine how much it means to me that you use the recipes I publish. The Onion soup was quite the surprise as I really have burned more onions on the road to caramelization than I can count. We are breathing a huge sigh of relief here today. And you should have heard the cheering and honking here in New York City. He got only 24 percent of the vote here. We are all praying for a return to some kind of normal human behavior. Enjoy your sister-in-law’s visit. We are ever vigilant and reports of New York’s demise are way off the mark. Let me know what you think of the recipes you’ve selected. All best, Stay Safe. Stay Strong. This too shall pass.

      • Dear Monte,

        The soup was splendid indeed! A smash hit! Thanks!!!

        The outcome of the election made us VERY happy! Yay Joe and Kamala!!!
        If we could’ve been dancing in the streets along with so many of our American friends, we would’ve been! But we have been and still are experiencing extreme whiteout blizzard conditions, and therefore NOT conducive to any outdoor dancing! 🥶

        Can’t wait to try the other recipes of yours on my roster. This week!!!



        • Dear Kate, So happy the soup was a success. And isn’t really worth the time? Yes, we had an absolutely astonishing day in New York yesterday. The outpouring of happiness and relief was incredible. When announced we heard tremendous cheers as people opened their windows and welcomed this victory for decency and common sense. This went on for hours. Then last night’s speeches reminded us of the last time we had a real President and not a failed Game Show Host. I cannot you are in a blizzard while it’s 72 here and sunny! Best of luck with this week’s menus! Always so great to hear from you! Monte

  • Hi Monte—This was absolutely the BEST onion soup I have ever tasted! The flavors are so concentrated and delicious, but so easy to make! Thank you! Happy holidays!

    • Dear Marie, Thank you so much for taking the time to write. I couldn’t be more pleased that you like the soup. It really is a keeper. As we prepare for a major storm to hit New York City in the next couple of days, I am very pleased that the freezer is home to this soup. Merry Christmas and happy holidays to you too! Warm regards, Monte

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