Slow Cooker French Onion Soup

Slow Cooker French Onion Soup is a simple soup of caramelised onions with a rich beef stock. It is easy to make in the slow cooker and the ingredients are inexpensive and freely available.  Top each bowl of soup with a grilled cheese crouton and you have a filling and delicious meal.

Slow Cooker French Onion soup in bowls with croutons

Why make French Onion Soup?

French Onion Soup is a classic recipe that is still the favourite of French citizens and is served in restaurants and bistros across Paris. Like all classics, its enduring legacy is simply because it is so good!

Unlike many classic recipes, it is actually really easy to make at home. You need to cook the onions slowly in butter and caramelise them with sugar, then a good beef stock is essential. There are only a few ingredients so it’s worth getting the best. The resulting soup won’t disappoint you and it’s ideal to serve elegantly for a dinner party or in a more rustic style for a casual lunch.

Why should I make French Onion Soup in my Slow Cooker?

The key to good French onion soup is the long slow sauteeing of the onions in butter. Your slow cooker is ideal to do this task. It keeps the heat low and cooks down the onions without allowing them to colour too much.

Soup really benefits from long slow cooking as the flavours develop and combine. A slow cooker guarantees a low simmering heat.

Convenient and Economical

Slow Cooker cooking is very convenient and economical. I know I can leave the soup cooking and go and do something else. You can cook this soup slowly on your stovetop. However as some of the liquid evaporates, you will have to stay close by to check the soup.

Slow Cooker French Onion Soup with bread and spoons

What ingredients do I need to make Slow Cooker French Onion Soup?

The ingredients for French Onion Soup are very simple and you will probably already have them in your fridge or larder. The magic of this soup is that those simple ingredients make a totally blissful bowl of soup. You will need:

For the Soup

  • onions
  • butter
  • plain (all-purpose) flour
  • beef stock
  • sugar
  • bouquet garni
  • salt
  • pepper

For the Croutons

  • French bread stick
  • Cheese
Onions

What kind of onions should I use for the soup?

Use standard brown (yellow) onions. Although these onions have a pungent taste when raw, they melt down to a mild rich flavour and are robust enough for the long slow cooking process.

What is the best way to slice onions for French Onion Soup?

It really depends on what you feel most comfortable using. Slice the onions with a sharp knife if you can slice them finely this way. I use a mandoline to slice the onions but I do use the hard guard because the blade is very sharp. You can also use a food processor with a slicing disk.

Slow Cooker French Onion Soup two bowls with spoons

What kind of stock can I use?

Use good quality beef stock, most supermarkets sell ready-to-use beef stock. You can use concentrated stock pots and if it’s just an everyday soup on a budget, then you can use stock cubes. Homemade beef stock takes a long time to make but if you want to try then Delia Smith has an excellent recipe for Beef Stock.

Can I use vegetable stock?

You can use vegetable stock, however the soup will not have the same rich taste. You can boost the flavour by adding a little vegetarian Worcestershire Sauce or Mushroom ketchup to the soup.

Bouquet Garni herbs on muslin

What is a bouquet garni?

A bouquet garni is a bundle of herbs either tied with string or tied inside a muslin bag. If you tie the herbs together they tend to disintegrate and pieces will remain in your dish. I prefer to put the herbs onto muslin and tie the herbs inside. Muslin is a loosely woven fabric and it allows the flavours of the herbs to go into the soup without any pieces escaping.

For the French Onion Soup I use a classic bouquet garni of a bay leaf, sprigs of thyme and parsley stalks.

baguette french stick

What kind of bread can I use for the croutons?

Use slices of baguette (French Stick) for preference. The slices of bread are topped with grated cheese and grilled. I like to grill the cheese before placing it on top of the soup bowl. Alternatively, you can put the bread and cheese onto the soup and put the whole bowl under a hot grill.

If you don’t have a french stick use another artisan-style bread like sourdough or even a crusty bloomer. When you eat the soup push the cheesy bread down into the soup, soaking up all the sweet oniony broth.

I don’t have any Gruyere cheese, what can I substitute?

I use Gruyere cheese because it melts beautifully and has a great flavour. Another French favourite is Comte cheese. However, you can use cheddar cheese or any other cheese that melts well. If you like long strings of mozzarella then go for it!

Fridge and Freezer

Can I make Slow Cooker French Onion Soup ahead of time?

Make this soup in your slow cooker, then store it in the refrigerator for up to 3 days. Reheat the soup until piping hot and serve with the cheese croutons.

Can I freeze French Onion Soup?

French Onion Soup freezes very well. Let the soup cool completely in a sealed freezer-proof container in the fridge. Freeze for up to 6 months, defrost in the fridge overnight and reheat until piping hot before adding the cheese croutons.

Vegetarian

This recipe contains beef stock which is not suitable for vegetarians. It is possible to substitute vegetable stock however the soup will not taste the same.

Gluten Free

French onion soup contains a little wheat flour. You can leave this out or substitute cornflour (cornstarch) or GF flour. If you use ready-made stock, stock pots or cubes please check the ingredients to ensure the stock is safe for you to consume.

For more information regarding any dietary information provided on this website, please refer to my Nutritional Disclaimer.

Slow Cooker French Onion Soup with spoon

How should I serve Slow Cooker French Onion Soup?

Serve French Onion Soup in small bowls as an elegant starter or appetiser for a formal meal. You may wish to make small cubed croutons if you are serving other courses, as the large croutons are very filling. I like to serve French Onion soup for lunch or supper as a hearty and casual meal. The caramelised onions in rich beef broth and the delicious grilled cheese crouton make it a warming and filling dish.

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I’m a big fan of a warming bowl of soup. Check out all my soup recipes for more bowls of deliciousness.

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Slow Cooker French onion soup in bowls with cheese croutons

PIN FOR LATER

Slow Cooker French Onion Soup in bowl with cheee crouton

Slow Cooker French Onion Soup

Janice Pattie
Slow Cooker French Onion Soup is such a simple soup to make, and the ingredients are inexpensive and freely available.  It’s an easy version of the classic French recipe and it’s packed with flavour. Top with a grilled cheese crouton and you have a filling and delicious meal.
5 from 2 votes
Prep Time 10 minutes
Cook Time 5 hours 10 minutes
Total Time 5 hours 20 minutes
Course appetiser, Soup
Cuisine French
Servings 6
Calories 411 kcal

Equipment

  • 1 Slow Cooker
  • 1 Sharp knife or mandoline or food processor with slicing disc
  • 1 chopping board
  • 1 weighing scales
  • 1 Measuring spoons
  • 1 Square of muslin
  • 1 piece of string
  • 1 Air Fryer optional

Ingredients

For the Soup

  • 4 large onions
  • 80 grams (6 tbsp) butter
  • 2 tsp caster sugar superfine sugar
  • 2 tbsp plain flour all purpose flour
  • 2 tsp salt
  • 1 litre (4.25 cups) beef stock beef broth
  • 1/2 tsp pepper

For the bouquet garni

  • 1 (1) bay leaf
  • 10 grams (10 grams) fresh thyme on the stem
  • 10 grams (10 grams) parsley stalks

For the croutons

  • 6 slices (6 slices) French stick
  • 100 g (0.75 cups) Gruyere cheese grated

Instructions
 

For the Soup

  • 4 large onions, 80 grams butter
    Put the onions and butter into the slow cooker put on the lid on HIGH and cook for 2 hours.
    French onion soup onions and butter in slow cooker
  • 2 tsp caster sugar
    Add the sugar and mix through the onions. Cook for a further 2 hours.
    French onion soup caramelised onions with wooden spoon
  • 2 tbsp plain flour
    Add the flour and mix the flour into the onions and butter. cook for a further 10 minutes.
    Add flour to French onion soup
  • Make the bouquet garni by placing a sprig of thyme, bay leaf and parsley into a piece of muslin and tie with string.
    Bouquet Garni herbs on muslin
  • 1 litre beef stock, 1/2 tsp pepper, 2 tsp salt
    Add the beef stock, season with the salt and peper and add the bouquet garni. Cook for a further 1 hours.
    Add beef stock to french onion soup in slow cooker
  • Remove the bouquest garni.
    Remove bouquet garni from French onion soup

For the Cheese Croutons

  • 6 slices French stick, 100 g Gruyere cheese
    Cut the bread into four thick slices, top each slice with grated cheese.
    French bread with grated cheese
  • Place under a preheated grill (broiler) and grill until the cheese is melted and bubbling. Alternatively, place the slices of bread into a pre-heated Air Fryer and top with the grated cheese. Cook at 200F for 10 minutes.
    Grilled cheese on french bread
  • Fill the bowls with the piping hot soup and top each with a cheese crouton. Serve immediately.
    Slow Cooker French Onion Soup in bowl with cheee crouton

Nutrition

Calories: 411kcalCarbohydrates: 47gProtein: 17gFat: 18gSaturated Fat: 10gPolyunsaturated Fat: 1gMonounsaturated Fat: 5gTrans Fat: 0.4gCholesterol: 47mgSodium: 1705mgPotassium: 552mgFiber: 4gSugar: 8gVitamin A: 682IUVitamin C: 7mgCalcium: 283mgIron: 5mg
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First Published January 2014.

11 Comments

  1. well firstly let me just say that your blog looks so cool, well done honey, I know how hard it is to take that leap and redesign but you've done it so well and it looks just great… I need to get my act together too!… secondly, wow, that dark, rich onion soup… glorious!… xx

    1. Thank you, Chloe. It really is the most delicious soup and caramelising the onions in the slow cooker is a game changer.

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Recipe Rating




5 from 2 votes