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Easy Lacto-Fermented Onions Recipe

Lacto-fermented onions are seriously one of my all-time favorite ferments! For starters, they are delicious and smell amazing! Fermented onions in a salt brine will keep for a very long time, so this is a great way to keep onions on hand all the time (they last forever), and they are so, so, sooooo versatile!

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Lacto Fermented Onion Slices

Fermented onions and pickled onions are not the same. Pickled onions typically uses hot water, white vinegar or apple cider vinegar to preserve the onions. While vinegar is acidic and does a great job at preservation, it bypasses the fermentation process which naturally produces its own acidity and preservatives. In addition, vinegar preservation is generally considered a “dead” food, and this process does not increase nutrients naturally found in the raw onions. The pickling process results in “pickled onions” that can be ready by the next day, but the fermenting process typically takes longer to complete.

Some people choose to ferment in a mason jar, but I personally find that using Fido jars with an airlock produces the best results when fermenting foods. A Fido jar is with an airlock attachment (fermentation lid) is an airtight container that will lock out oxygen while still allowing CO2 (carbon dioxide) to escape. This eliminates the need to “burp” your containers, as the pressure within the jar cannot build up enough to cause the jar to explode. These jars also quickly reduce the pH of the salt water brine and foods so that the good bacteria can grow and your jar of probiotic goodness can have a very long shelf life.

How do I prepare onions for fermenting?

When you are fermenting onions, you have several choices. First you’ll want to peel your onions. After that, you can decide to:

  • slice them
  • dice them
  • quarter them
  • ferment them whole

The sky is the limit.

Why make lacto-fermented onions?

If you’ve tried onions in all its forms, then you know that the taste of raw onions can be quite spicy. When you ferment onions, it eliminates the harsh bite you can experience from onions. In my opinion, fermentation is the most delicious way to prepare onions! I even have a fermented red onion recipe!

Fermenting onions whole is a great way to preserve for a year or more, or to have fermented onions ready that you can decide what to do with them whenever the need arises. It’s also a great way to ferment pearl onions, which are small and already bite-sized.

For this fermented onion recipe, we will slice the onions thinly as if making sauerkraut.

For thin slices, I love to use the smaller side of this mandoline. I’ve had this mandoline for around 7 or 8 years and still love it as much as the day it arrived! The thin shreds it creates is perfect for making homemade sauerkraut and fermented onions.

I use these thin slices on everything. It’s amazing as onion relish on a sourdough Tuna sandwich, amazing by itself, and amazing in salads!

Related Post: Fermenting Equipment I LOVE And Use Regularly

How long does it take to ferment onions?

Fermenting onions is super easy and quick if you slice them thinly. I usually give my thinly sliced onions 2 weeks to ferment (one week at room temperature, and one week in the fridge), but shortening this time by about half is OK, and you can generally consume after a week.

If you are fermenting whole onions, it is better to give them the full 2 weeks, and 4 weeks would be even better. Make a judgment call based on if you’re using large onions, cutting them in half, etc. You’ll also need a salt water brine if you are using whole onions.

To make the brine solution, mix 5 grams of salt in a cup of water, and cover the onions fully. Using a fermentation weight can be helpful when using whole onions since they may try to float. Floating vegetables can increase the likelihood of kahm yeast and mold development. I generally don’t use a weight when I am making my thinly sliced onions. I do leave a few inches of empty headspace in the jar, however, because it produces plenty of liquid!

Can I add spices to my fermented onions?

Even though this recipe only contains the basic ingredients for fermenting onions (onions and pure sea salt), I want you to know that fermented onions are very versatile and you can add a large variety of herbs and spices to make it super yummy. Don’t get me wrong, fermented onions are delicious as they are, they have a subtle sweetness and don’t get as sour as many fermented foods do.

You can play around with adding:

  • a few garlic cloves (whole, sliced, chopped or minced)
  • fresh herbs such as thyme, rosemary, basil
  • habanero or jalapeño peppers
  • dried chili peppers and/or bay leaf
  • a tbsp or two of red pepper flakes
  • a handful mustard seed
  • a few allspice berries
  • fresh vegetables such as carrots, beets, celery (grated or cut small)

These can add a variety of flavors and colors to your onions, increase the health benefits and making it even more tasty!

Do I need to use a starter culture when fermenting onions?

As with pretty much all my recipes, you do not need to use a starter culture when fermenting onions.

Some people like to use whey or sauerkraut brine to “jump-start” the fermentation process, but I highly recommend NOT doing that. Using a starter causes your ferments to bypass important steps. It can also lead to mushy or slimy end product. The already present bacteria on the onions are enough to fuel fermentation and the natural sweetness from the onions will help the beneficial bacteria grow.

Besides the special anaerobic jars I recommend for fermenting, everything you need to create delicious fermented foods is likely already in your home kitchen!

What can I eat fermented onions with?

I love to throw them in salads, on sandwiches, in Chipotle bowls store-bought or homemade, in fajitas, tacos… it would probably be easier to say what I don’t enjoy them with!

My son really loves fermented onions as well. He like to eat them alongside his eggs, with potatoes, in grain-free tacos (I finally found some great options for him!), and by themselves.

But there are many other ways to use up your fermented onions, and I bet you’ll find them disappearing VERY quickly.

Use them on:

  • hamburgers
  • hot dogs
  • avocado toast

Let’s go ahead and learn how to ferment onions so you can enjoy them, too!

Fermented Onion Rings

Easy Lacto-Fermented Onion Recipe
Fermenting onions is easy, quick, and makes the most perfect, thinly sliced condiment for salads, sandwiches, soups, etc.
Course Ferment


  • Mandoline or knife
  • 1 L jar
  • Airlock lid (I use Pickl-It) recommended
  • mixing bowl
  • Kitchen scale


  • 2 lb onions (approximately 2 large onions)
  • 8 g salt


  • Peel onions and remove the ends.
  • Put mixing bowl on kitchen scale and zero it out.
  • Slice onions thinly and move to the mixing bowl. Using a mandolin will help create uniformly thin slices.
  • Note measurement of sliced onions in mixing bowl. It should be around 1.5lbs. If you have more than this, remove the extra. You can use the rest in dinner, a salad or refrigerate/freeze for future use.
  • Add salt (approximately 8g or 1.5 tsp) and mix well.
  • Move onions to jar, pressing down as you go along. Make sure your jar is clean and you don't fill past the shoulder. Onions tend to create plenty of juice so making the cut off the shoulder or a little below is pretty wise!
  • Let ferment for one week at room temperature, and one week in the fridge. This is a ferment you will want to put on something to catch excess liquid just in case it leaks!
  • Enjoy!!!
Keyword Onions

This is a fermenting recipe I highly recommend you try. It is probably one of the easiest ferments to make, so it’s a great ferment recipe for absolute beginners.

Using weights is optional for the slices, but if you are new to fermenting you may want to go ahead and weigh them down. I find that within a day or two there’s plenty of brine to completely cover the onions.

These lacto-fermented onions make the perfect condiment. Once you’ve tried them, let me know if you agree!

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Tuesday 5th of September 2023

Hi, How long do they last in the fridge? Do they need to in brine in the fridge? My opinions appear moist but not under a lot of brine. I it safe?

14 Non-Dairy Fermented Foods You Must Try - Nourishing Time

Thursday 30th of June 2022

[…] benefits of natural probiotics at all. You can make your own fermented beets, green beans, garlic, onions, ginger and more. You never have to use whey to ferment your vegetables or sodas, nor should […]


Friday 21st of May 2021

WOW just tasted the onion ferment and it is great. Made it in my Mad Millie Crock as per your recipe and it is a hit.


Wednesday 26th of May 2021

I am so glad you enjoy it! It is definitely one of my favorites and is an easy one to make!


Wednesday 19th of May 2021

Hi, I am wondering if we need to add water to cover the onion?


Monday 8th of November 2021

@Jer, It's important the the onions are under the brine. If they don't release enough liquid, you can add brine to cover, 1/2 tsp salt to a cup of water.


Wednesday 19th of May 2021

Hey there! If you thinly slice the onions then there is no need to add water. It will create it's own brine.

If you cut in quarters or chunks then you would cover with a 2% brine.

Naturally Fermented Red Onions | Nourishing Time

Tuesday 27th of October 2020

[…] as with the previous recipe I shared, you are welcome to cut your onions however you like. I tend to shred them, but you have […]