Slow Cooker French Onion Soup

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Let your appliance do all the hard work to make this spectacular slow cooker French onion soup! It’s rich and flavourful, and topped with a giant cheesy crouton.

A bowl of French onion soup topped with a crispy cheese crouton.

If you like it when your dinner cooks itself, you’ll love this slow cooker French onion soup. It’s rich and earthy, sweet and tangy, and all-round wonderful. Add a super cheesy crouton, and you may as well just marry it now.

A bowlful of French onion soup.

Slow cooker soup

I’m so excited to get my slow cooker out of the cupboard, now that the weather’s cooling off. There’s nothing better than an appliance that takes a job off my hands (notable mention to my other best buddy, the dishwasher).

A slow cooker is the perfect low effort appliance. Just chuck in a few raw ingredients (which in this case literally took me about 8 minutes), leave it to cook throughout the day, and dinner’s made itself. Hurrah!

Soup is the perfect thing to make in a slow cooker, as whatever you add has plenty of time to cook down beautifully, intensifying the flavours. Often, the amount of time you cook soup for isn’t hugely important, so if you’re not quite ready to eat when the soup’s ready, it probably won’t mind simmering away for another hour or two, until you’re hungry.

A bowl of slow cooker French onion soup.

What is French onion soup?

This time, I used my slow cooker to make French onion soup. If you’ve never had French onion soup before, you can probably guess what it is: an onion soup that originated in France, way back in the 18th century.

French onion soup is incredible. It’s a brothy soup, with plenty of pieces of soft, sweet caramelised onions throughout. The broth is intensely flavourful – sweet and earthy in equal measures.

And best of all… French onion soup is often served with a thick, gooey, cheesy crouton. Just in case you thought it couldn’t get any more wonderful.

How to make slow cooker French onion soup

Raw onions and herbs in a slow cooker pot.

Step 1: Add the soup ingredients to the slow cooker

Before I get too carried away with that cheesy crouton, we need to make the actual soup. Obviously the main ingredient is onions, but there are a few other ingredients to add to the slow cooker too, which all combine to make a seriously tasty broth:

  • garlic
  • vegetable stock
  • dried thyme
  • black pepper
  • balsamic vinegar
  • a little sugar
  • butter

It all melts together, along with the juices from the onions, and creates an incredible soup.

Raw sliced onions in a slow cooker.

Step 2: Cook low and slow

Give everything a good mix, and leave it to cook in the slow cooker. I prepared my soup after breakfast, and it was ready to eat by dinner time – about 9 hours. The long cooking time gave the onions a beautiful deep brown colour, and really intensified their flavour.

If you don’t have as much time on your hands, it will be fine to eat after about 6-7 hours, but it won’t be quite as rich in colour or flavour.

Rich slow cooked French onion soup in a pot.

You can eat French onion soup on its own (which is still really tasty!) – if that’s what you’d like to do, you’re already done. Seriously easy, right?

However, I’d highly recommend doing one more thing…

French onion soup topped with a slice of French bread and grated cheese.

Step 3: Make the cheesy crouton

The enormous, cheesy crouton that you often find on top of a bowlful of French onion soup is the perfect way to finish off the recipe.

The underside of the crouton soaks up a little of the broth, while the top is covered with a blanket of gooey melted cheese.

Traditionally, the crouton is grilled right on top of the soup. As long as your bowl is heat-safe (don’t use a plastic bowl, for example!), you can heap the bread and cheese right on top, and stick it straight under the grill (broiler).

I’d recommend allowing the grated cheese to cascade down over the side of the piece of bread, rather than being too tidy with it… the end result is like an avalanche of molten cheese that pools in the soup and makes it extra yummy.

A bowlful of French onion soup topped with a cheesy crouton.

What cheese should you use for French onion soup?

Traditionally, the crouton on a bowl of French onion soup uses French cheese – usually either Comté or Gruyère.

To be honest though, I don’t like buying expensive ingredients that I only need for one recipe – so I just used a bit of good old cheddar instead. It’s very hard to go wrong with a cheesy crouton, so it really won’t matter.

Just use whatever cheese you have on hand. It’ll be tasty whatever you go for.

French onion soup made in the slow cooker with a golden brown cheesy crouton.

What if I don’t have any heat-safe bowls?

No problem! If your bowl won’t survive under the grill, just cook the cheesy croutons on a baking tray instead, then pop them on top of a hot bowl of soup. You won’t get quite the same effect, with the cheese overflowing into the soup, but it will still be delicious.

A spoon taking a scoop of French onion soup.

Can I cook French onion soup in advance?

Yep! This slow cooker French onion soup reheats really nicely, so feel free to cook it in advance of when you need it. I know not everyone has 9 hours to spare before they want to eat each day!

To reheat, you can either bring the whole batch to a simmer in a pan on the stove, or microwave one bowlful at a time. Don’t add the cheesy crouton until right before you’re ready to eat, otherwise it will get too soggy underneath.

A spoon taking a scoop of French onion soup.

Slow Cooker French Onion Soup

Let your appliance do all the hard work to make this spectacular slow cooker French onion soup! It's rich and flavourful, and topped with a giant cheesy crouton.

If you’ve cooked this recipe, don’t forget to leave a star rating!

4.58 from 7 votes
Print Pin Comment
Prep Time: 15 minutes
Cook Time: 9 hours
Total Time: 9 hours 15 minutes
Servings: 2 people
Calories: 488kcal
Author: Becca Heyes


For the slow cooker French onion soup:

  • 4 large onions sliced
  • 30 g (~ 2 tbsp) butter, roughly chopped
  • 1 tbsp caster sugar (optional)
  • 1 tbsp balsamic vinegar
  • 2 tsp minced garlic or garlic puree
  • 1 tsp dried thyme
  • 1 tsp black pepper
  • 750 ml (~ 3 cups) vegetable stock

For the cheesy crouton:

  • 2-4 thick slices French baguette
  • 2-4 tbsp grated cheese (I used cheddar)


  • Add all of the soup ingredients to a slow cooker, and mix well to break up the onions. Cook on high for approximately 9 hours, or until the onions are a deep, rich brown colour, and very soft.
  • Serve on its own, or with a cheesy crouton. To make the cheesy crouton, dish up a bowlful of soup into a heat-safe bowl, and top with 1 or 2 thick slices of French baguette. Sprinkle over plenty of grated cheese, then place under a grill (broiler) for a few minutes, until the cheese is melted and slightly crispy. Serve immediately.


Nutrition Facts
Slow Cooker French Onion Soup
Amount Per Serving (1 bowl)
Calories 488 Calories from Fat 181
% Daily Value*
Fat 20.1g31%
Saturated Fat 12.2g61%
Cholesterol 53mg18%
Sodium 536mg22%
Potassium 557mg16%
Carbohydrates 65.1g22%
Fiber 8.1g32%
Sugar 20.1g22%
Protein 14.8g30%
Calcium 258mg26%
Iron 4mg22%
* Percent Daily Values are based on a 2000 calorie diet.

Nutritional information is approximate, and will depend on your exact ingredients. Please calculate your own nutritional values if you require accuracy for health reasons.

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Recipe Rating


  1. This recipe is awesome. I’ve made it 3 times, followed recipe except used beef broth. Great results everytime.5 stars

  2. Are you absolutely sure you used raw onions straight in the slow cooker in your recipe? I doubt it, mine’s been cooking for two days and the onions are still stringy and broth is thin, not rich and gloopy like your illustration – even an extra 2 hours in the oven hasn’t helped much – very disappointed to say the least. Slow cookers don’t cook raw onions properly – I should have fried the onions in butter and oil for 10-15 mins before putting them in the slow cooker with the other ingredients. I knew this from past experience but gave your recipe the benefit of the doubt – now I know I was right to be sceptical! I’d be interested to see other reivews – maybe it’s my slow cooker – what brand do you use, please?2 stars

    1. @Becca @ Easy Cheesy Vegetarian, the onions cooked they just never get hot enough to soften like they do by gently frying them first, they retain almost a slight crunch and don’t break down enough to thicken the soup. There are lots of comments online on the subject and consensus of opinion seems to be that slow cookers don’t do onions justice – that’s why I wondered which brand you used? I’m going to try this again but with softened onions next time! :) PS love your Mushroom Bourguignon!!

      1. I cooked this today as put a message on your FB group with pic. Absolutely gorgeous, and we’ll be doing it again for sure!!5 stars

    1. Sorry Tina, I’m afraid I’m not very familiar with gluten free flours, so I can’t really advise there. The flour is only for thickening though (it’s not like it’s a bread recipe or anything), so I imagine any neutral-tasting flour would work ok!

  3. Hi,

    thank you so much for this recipe, we enjoyed this soup very much. I have made an onion soup before and it took ages to do. I love slow cooker it made it so much easier.

  4. Too Sweet. Thank you for the recipe however, I didn’t know if I was eating desert or dinner. In all fairness I do not have a sweet tooth- however my husband does and he was the first to mention that it was too sweet. Next time I will leave the 2 tablespoons of sugar out. Happy Cooking.