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.

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.

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

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.

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.

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.

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!

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.

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.

More Traditional Soup Recipes

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

Scottish Chicken and Leek Soup is a traditional soup that is also known as Cock-a-Leekie. Made with leeks and chicken stock with a little rice, this is a simple and very delicious recipe that you can make on the stove, in your Slow Cooker, or in a pressure cooker.

Easy Scottish Red Lentil Soup is a hearty and warming soup. I’ve taken this traditional Scottish soup recipe and made it even easier to make. There are only five ingredients and one of them is water!

Cullen Skink is a traditional Scottish soup made with smoked fish, potatoes, and onions. It’s the perfect soup to serve as a hearty family meal with homemade soda bread and equally at home at an elegant dinner party.

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First Published January 2014.