Slow Cooker French Onion Soup

My slow cooker French onion soup combines deep, caramelized flavors with an effortless cooking process. I remember the first time I tried French onion soup in a local neighborhood bistro, and I was hooked by the caramelized onions, rich broth, and that golden cheese topping. Now, I make this slow cooker version that lets the onions caramelize all day while I do literally anything else. It’s cozy, and just as satisfying on a weeknight as it is for entertaining friends.

This post contains helpful tips and tricks! If you’re in a rush, please use the “Jump to Recipe” above!

💭 The Secret to Perfect Crockpot French Onion Soup

Making French onion soup in the slow cooker has been a game-changer for me. I tried so many recipes that still wanted me to caramelize onions on the stovetop first—and I found myself wondering, what’s the point of the slow cooker then? So after a few rounds of testing, I finally landed on this version.

EVERYTHING happens right in the slow cooker, from the slow, golden caramelization of the onions to the rich, savory broth that develops over hours. It’s the kind of recipe that feels luxurious without the effort, and honestly, it’s spoiled me for any other method. Why stand over a pan when you can let your slow cooker do all the heavy lifting?

Want other delicious slow cooker recipes? Check out my slow cooker green bean casserole, slow cooker tortilla soup, or slow cooker spaghetti squash.

📋 Ingredients and notes

To make this slow cooker French onion soup, you’ll need yellow onions, garlic, vegetable broth, butter, gruyere cheese, soy sauce, balsamic vinegar, sugar, bay leaves, fresh thyme, salt, and pepper. Optionally, you can add dry sherry wine.

An overhead image of the ingredients of French onion soup.

Variations & Substitutions

Onions. I like to use yellow onions because they have the perfect balance of sweetness and depth when caramelized. You can use white onions for a milder flavor, or even a mix of yellow onions, sweet onions, and shallots.
Vegetable broth. You can also use mushroom broth, or make your own homemade vegetable stock.
Balsamic vinegar. Substitute with red wine vinegar or sherry vinegar.
Soy sauce. Substitute with tamari or Worcestershire sauce.
Sugar. You can also use brown sugar, honey, or maple syrup.
Dry sherry. If you don’t have sherry, substitute with white wine or even apple cider vinegar for a similar depth of flavor.
Baguette. You can also use sourdough slices, croutons, or crostini.
Gruyère cheese. You can also use Swiss cheese, Emmental, or a mix of parmesan and mozzarella.
For a spicy kick, add a pinch of red pepper flakes or a dash of hot sauce to the broth.
If you want a creamier soup, stir in a splash of heavy cream or cashew cream at the end.

Make vegan French onion soup

For a vegan version, use olive oil, skip the cheese, and top with crispy fried onions for a crunchy garnish.

📖 How to make Crockpot French onion soup

Step 1:
Set your slow cooker to high. Add melted butter (or olive oil), sliced onions, and sugar (if using). Stir to coat the onions evenly. Prop the lid open slightly to allow steam to escape and cook for 6-8 hours on low, stirring occasionally, until onions are golden brown and soft.

An overhead image of onions in a slow cooker.

Step 2:
Once onions are caramelized, add minced garlic and cook for another 2-3 minutes on low, stirring occasionally.

An overhead image of adding garlic to the slow cooker.

Step 3:
Add balsamic vinegar and sherry to deglaze the slow cooker. Stir well to pick up any browned bits.

An overhead image of adding balsamic vinegar to the slow cooker.

Step 4:
Add vegetable broth, thyme, bay leaves, soy sauce, salt, and pepper. Stir, cover, and cook on low for another 4-6 hours.

An overhead image of adding fresh thyme, bay leaves, and other seasonings to the slow cooker.

An overhead image of adding vegetable broth to the slow cooker.

Step 5:
Preheat the oven to 400°F (200°C). Slice the baguette and toast until golden and crispy.

Step 6:
Remove thyme sprigs and bay leaves from the soup. Ladle the soup into oven-safe bowls, top with toasted baguette slices, and sprinkle with shredded Gruyère.

An overhead image of adding sliced baguette to the soup.

An overhead image of adding gruyere cheese to a bowl.

Step 7:
Place the bowls under the broiler for a few minutes until the cheese melts and is bubbly. Serve with extra thyme or a drizzle of balsamic reduction if desired!

An overhead image of French onion soup in a bowl with baguette slices and melted cheese on top.

Shruthi’s Top Tip

Prop the slow cooker lid open slightly during the first step to allow steam to escape and concentrate the flavor of the onions. Stir the onions occasionally during the caramelization phase to prevent sticking or uneven cooking.

Slice onions evenly to ensure they cook at the same rate and caramelize uniformly.
Use a splash of water if the onions start to stick too much during caramelization, loosening any browned bits.
Taste and adjust the seasoning after cooking—vegetable broths vary in saltiness, so you may need less or more salt.
Use freshly shredded Gruyère cheese for the best melt and flavor. Pre-shredded cheese doesn’t melt as smoothly due to anti-caking agents.
Toast baguette slices thoroughly to ensure they stay crisp under the broiler.
Broil the cheese-topped soup bowls on a baking sheet to catch any drips and make cleanup easier.

👩🏽‍🍳 Troubleshooting FAQs

What if my onions aren’t caramelizing evenly?

Stir them every hour and ensure the lid is slightly open to let steam escape. Adding a splash of water can also help deglaze stuck bits.

Can I skip the sherry?

Yes, but it adds a layer of complexity to the broth. Substitute with white wine, apple cider vinegar, or just leave it out.

What’s the best way to toast the baguette slices?

Brush lightly with olive oil and bake at 400°F until golden and crispy. This prevents them from getting soggy in the soup.

Can I use a different cheese?

Absolutely! Swiss or fontina are great substitutes if you don’t have Gruyère.

🍴 How to serve French onion soup

French onion soup is hearty and rich, so I love pairing it with lighter sides for a well-rounded meal. If you don’t have sliced baguette, you can serve it with garlic bread or savory puff pastries (even better if they’re filled with cheese or mushrooms – like a mushroom tartlet). You can also serve it with a vegan Caesar salad, roasted vegetable salad, or stuffed mushrooms.

An overhead image of slow cooker French onion soup.

🍴Storage and reheating suggestions

To store leftover French onion soup, store it in an airtight container in the fridge for 3-4 days. Make sure to remove the bread and cheese on top before storing. The bread will become soggy, and the cheese can clump in the soup. You can store these separately or prepare fresh toppings when reheating.

French onion soup freezes well. Transfer it to a freezer-safe bag and store it for up to 3 months.

To reheat, warm the soup on the stove over medium heat or place it in the microwave for 2-3 minutes.

🍴 More slow cooker recipes

If you like this recipe, check out my other slow cooker recipes:

HPVE

2 Comments

Ruth G says:

February 12, 2025 at 6:40 am

I think you need to change the prep times and cook times. I want to make this when I have HOURS to monitor the slow cooker. It sounds so good and I love French onion soup.

1. Shruthi Baskaran-Makanju says:
  
  February 12, 2025 at 8:48 am
  
  Hi there! The slow cooker time is separately called out, but it was not reflecting in the “total” time because it’s not actually active cook time (I often dump it in overnight and then just check on it the next morning).