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There is nothing quite like a big bowl of French onion soup on a chilly evening. Sweet, caramelized onions, rich, savory beef broth, and a layer of bubbly melted cheese on top. What sounds cozier than that?
The only problem with the classic version? Caramelizing onions on the stove takes forever and a whole lot of stirring. Good news! This slow cooker French onion soup does the work for you. Just 15 minutes of slicing, and your crockpot slowly turns a pile of onions into the most flavorful soup base. Your house will smell amazing all day long.
It’s naturally paleo-friendly, egg-free, and nut-free, and you can serve it light on its own or go all in with toasted gluten-free bread of choice and gruyere. Either way, this one is a cool-weather soup our house loves.
Why You’ll Love This Crockpot French Onion Soup
- Hands-off caramelized onions. No need to stand over the stove for an hour. The slow cooker does the caramelizing for you.
- Only 15 minutes of prep. Slice, toss, and walk away.
- Rich, restaurant-style flavor. A splash of balsamic and a tiny bit of fish sauce give it that deep, savory flavor you’d expect from a bistro.
- Easy to customize. Keep it light, make it cheesy, and go fully gluten-free with your favorite bread.
- Freezer-friendly. Make a big batch and stash some in the freezer for those busy weeknights.
What’s the Difference Between Onion Soup and French Onion Soup?
Onion soup is a broad term for any soup where onions are the feature. French onion soup is a specific style: onions are slowly caramelized until deep golden, then simmered in beef broth and traditionally topped with bread and melted gruyere. That long, slow caramelization gives French onion soup its signature sweet, rich flavor, and it’s exactly what the slow cooker does best.
Ingredients You’ll Need
*Scroll down to the recipe card for the complete list of ingredients and details.
- Onions: We use a mix of white and red onions. The white onions bring that classic savory onion flavor, and the red onions add a little extra sweetness and color.
- Butter and olive or avocado oil: The butter adds richness, and the oil keeps the butter from burning.
- Coconut sugar: Just a tablespoon helps the onions caramelize and balances the flavor.
- Garlic: Fresh minced garlic for depth.
- Balsamic vinegar: Adds a subtle tang and deepens the onions’ color.
- Thyme and bay leaves: The classic French onion soup herbs. Dried thyme or fresh thyme sprigs both work.
- Beef broth: The soup’s backbone. Use a good-quality broth or bone broth (or a mix of both) for the best flavor.
- Fish sauce: Don’t skip it! You won’t taste fish, I promise. It just adds a savory, umami boost.
- Arrowroot flour: Lightly thickens the broth. Regular all-purpose flour works too if you aren’t gluten-free.
- Salt and pepper: To taste.
Optional Toppings:
- Bread: A thick slice of toasted bread is classic. Our homemade paleo sandwich rolls or gluten-free and nut-free rolls are perfect here, or use your favorite gluten-free baguette.
- Cheese: Gruyère is traditional, and we like to add a little mozzarella for that extra-melty cheese pull.
How to Make French Onion Soup in the Slow Cooker
- Add the onions. Add the butter and oil to your slow cooker, then add the sliced onions, coconut sugar, garlic, balsamic vinegar, thyme, bay leaves, and half the salt and pepper. Give everything a good stir.
- Slow cook on low for 6 hours. This is where the magic happens. There’s very little liquid at this stage, and that’s on purpose! It lets the onions soften, sweeten, and caramelize.
- Add the broth. Stir in the beef broth, fish sauce, arrowroot flour, and the remaining salt and pepper. Cook on low for 2 more hours.
- Remove the herbs. Pull out the bay leaves and any thyme stems, then taste and adjust the seasoning.
- Top and broil (optional). Ladle the soup into oven-safe bowls, top each with a slice of toasted bread and a generous layer of cheese, and bake until bubbly. Finish under the broiler for a minute to get that golden, crispy cheese.
French Onion Soup FAQ’s
Yellow or white onions are the classic choice because they have a balanced, savory-sweet flavor that holds up to long cooking. We love adding a couple of red onions for extra sweetness and a gorgeous deep color. Sweet onions like Vidalia work too, but your soup will lean sweeter, so you may want to skip the coconut sugar.
Yep! That’s the beauty of this recipe. There’s no need to sauté the onions first. Raw sliced onions go straight into the crockpot with the butter and seasonings, and over 6 hours on low, they slowly soften and caramelize all on their own.
A few simple tricks make a big difference:
Use a rich broth. Beef bone broth gives the most flavor.
Add fish sauce or Worcestershire. Just a little adds tons of savory depth.
Splash in red wine. Add a splash with the broth for a more classic bistro-style soup.
Don’t rush the onions. The full 6 hours is what builds that deep caramelized flavor.
It can be! On its own, this soup is about 163 calories per serving and is made with simple, real-food ingredients. The bread and cheese topping adds most of the extra calories, so if you want a lighter bowl, skip the topping or use just a sprinkle of cheese. It’s still cozy and delicious either way.
Absolutely. If you’re short on time, the Instant Pot makes rich French onion soup in under an hour. You’ll find the full Instant Pot directions in the recipe card notes below.
Tips for the Best Slow Cooker French Onion Soup
- Slice the onions evenly. Thin, even slices (about 1/4 inch) cook at the same rate, so nothing ends up crunchy.
- Use oven-safe bowls. If you’re broiling the cheese, make sure your bowls can handle the heat so they don’t crack.
- Swap the broth if needed. Vegetable or chicken broth both work, but the flavor won’t be quite as rich as beef broth.
- No arrowroot? All-purpose flour works if you don’t need the soup to be gluten-free.
Serving Suggestions
French Onion Soup is a delicious meal all by itself, but it can be fun to round it out with a couple of sides. Here are a few favorites:
- Easy Gluten-free Cornbread
- Gluten Free Garlic Bread
- Gluten Free Garlic Breadsticks
- Restaurant Worthy Caesar Salad Recipe
Can You Make French Onion Soup Ahead of Time?
Yes, and it actually tastes even better the next day! Make the soup through step 4, let it cool, and store it in the fridge. When you’re ready to eat, reheat it on the stovetop, then add the bread and cheese and broil right before serving.
How to Store and Freeze
- Fridge: Store leftover soup (without the bread and cheese) in an airtight container for 3–4 days.
- Freezer: Freeze in freezer-safe containers for up to 3 months. Thaw overnight in the fridge.
- Reheat: Warm on the stovetop over low heat, then top with bread and cheese and bake if you’d like.
More Slow Cooker Soup Recipes
Looking for more cozy bowls? Browse all of our Soups & Stews, or try one of these:
- Classic Slow Cooker Chicken Soup
- Slow Cooker Sweet Potato Chili
- Slow Cooker Short Rib Stew
- Gluten-Free Lasagna Soup
- White Chicken Chili
- Healthy Broccoli Cheddar Soup
- Turkey Wild Rice Soup
If you make this slow cooker French onion soup, leave a comment and rating below. We love hearing from you!
Watch the Video:
Slow Cooker French Onion Soup
Ingredients
- 3 tablespoons butter
- 1 tablespoon extra-virgin olive oil or avocado oil
- 4 white onions sliced
- 2 red onions sliced
- 1 tablespoon coconut sugar
- 3 cloves of garlic minced
- 1 teaspoon salt more to taste
- 1 teaspoon black pepper more to taste
- 1/4 cup balsamic vinegar
- 2 teaspoons dried thyme or 2 fresh thyme sprigs
- 2 bay leaves
- 7 cups beef broth
- 1/2 teaspoon fish sauce
- 1 tablespoon arrowroot flour
- Optional: red pepper flakes for serving
- Options: 1/4 cup shredded Gruyere cheese
- Optional: 1/4 cup shredded mozzarella cheese
- Optional: 1 batch bread see recipe here or here for nut-free
Instructions
- Place butter and oil in the bottom of your slow cooker insert.
- Add in sliced onions, coconut sugar, garlic, balsamic vinegar, half the salt and pepper, thyme and bay leaves. Toss to coat and cook on low for 6 hours.
- After 6 hours, add in beef broth, fish sauce, arrowroot flour, and the remaining salt and pepper.
- Let cook for the remaining 2 hours. Remove bay leaves and thyme sprigs if using fresh thyme. Taste and add salt and pepper as desired.
- Heat the oven to 350°F. Divide the soup between oven-safe bowls. Top each with a slice (or two) of toasted bread and sprinkle cheese in a thick layer over the bread and up to the edge of the bowl. Bake for 20 to 30 minutes until cheese is bubbling. Broil on high for 1 minute.
Can the fish sauce be substituted with something else?
We loved this recipe. I did substitute organic beef bone broth for the regular broth and did not add the sugar or arrowroot flour (didn’t have any). We also do not add the bread and used some Parmesan and mozzarella cheese in small amounts. It was so good and husband approved! Super easy. It was an interesting technique in the beginning with the crock pot cooking with very little liquid but it definitely worked. Thanks for another great recipe.
CAN THIS SOUP BE MADE WITH VEGETABLE STOCK? I’D LOVE SOME FRENCH ONION SOUP, BUT I’M A VEGETARIAN.
MANY THANKS,
LINDA DEMING
certainly.
Question: could you use this same method to make caramelized onions in the crock pot (ie just do the first step and don’t add broth, etc)? I always want caramelized onions for recipes but I’m too lazy to stand there and stir onions in a skillet for an hour.
Yes!
Could i substitute arrowroot powder with wheat flour?
Looks very tasty. I will have to try this at some point.
This sounds great! Love the idea of cooking it in a slow cooker – make everything much easier!
Hi Lexi, I’m wondering how I could convert this to make in my Instant Pot.. Any ideas? Thanks!
Made this tonight. It’s was delicious! My husbands first time ever trying French onion soup and he loved it. Next time I may add a splash of red wine, but totally not necessary. Thanks for another great recipe!
So glad you loved it!!
This is delicious!!! I made this in my large slow cooker this past weekend. I let it cook for 14 hours or so and then had it the next day. I didn’t bother with the bread and cheese, and just ate it as it is. Loved it! Thank you!
Hi Lexi. This looks amazing! I’m wondering if I could use chicken stock instead. I don’t eat beef at all so I’m wondering if this substitution would work. What are your thoughts? Thanks!
Hi Chris! It will work, but might not be as flagrant of flavors. Let us know if you try it!