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Fermented Red Onion Recipe

Pickled onions are usually preserved in vinegar these days, but traditionally they have been preserved by lacto-fermentation. With just a few simple ingredients – raw onions, salt, and water – you can have naturally pickled onions.

This fermented red onion recipe comes together quickly and while it uses red onions, you can easily substitute yellow or white onions.

red onion, mason jar, fermenting weight and lid on wooden table

Why Ferment Onions

Because they’re delicious!

The fermentation process transforms raw onions into milder onions that have a texture similar to cooked onions. They loose some of their spiciness (or bite) and are a great substitute for raw onions on sandwiches, wraps, burgers, avocado toast….or just out of the jar!

As with other fermented vegetables, fermented raw onions have added health benefits such as probiotics and enzymes that help with digesting them. Because of this people who have a hard time with raw onions can often enjoy fermented onions just fine.

Fermented foods have an acidic flavor, not as strong as vinegar pickled foods but acidic none the less. Also know that onions have sulfur compounds, the stronger the onions the more compounds. The smell of fermented onions is often stronger than other fermented foods, it’s not a bad smell, but it can be strong.

Which Onions to Choose

Any firm raw onion is a great choice from fermenting, this includes yellow, white, red, and Spanish onions.

My favorite way is to use red onions because they are sweeter than yellow or white and that pairs well with the acidity of the ferment. They also end up making a really pretty pink brine.

red onion slices on wooden cutting board with knife

How to Make Fermented Onions

There are a couple of ways to ferment onions. One way is by massaging salt into onion slices so that they release their natural juices and that creates the brine. The other way, is to pour a saltwater brine over the sliced onions. I usually choose to massage the salt into the onion slices. But both work great, so use whatever way you want.

Peel, rinse and slice the onions. I like to slice them very thin.

You’ll need to weigh the onion slices in grams in order to know how much salt to use. I’ll give a basic recipe in the recipe card but we all know that onions don’t come in exact sizes, so it’s nice to know how to calculate it for yourself.

Once you’ve weighed the onions in grams multiply that number by .02 (2%) to get the number of grams of salt you will need. Weigh the salt (different salts have different weights per volume; for example, finer salts will weigh more per tablespoon than coarse salts)

red onion slices in metal bowl being massaged with salt for fermenting

Put the red onion slices in a bowl and massage the salt into the slices for a few minutes. You’ll notice that the onions will release their juices (sweat) and create a brine in the bowl. The onions will break down and start to soften as you massage them. When the onions no longer feel crisp, they’re ready, this should take about 5 minutes.

You can add a few peppercorns or red pepper flakes to the onion slices for additional flavor, if you like.

Put the onion slices and the juice in a wide mouth pint size mason jar. Push the onion slices down into the jar. Insert a fermentation weight to keep everything under the brine and screw on a fermentation lid. If there is not enough brine from the onion juices you can top it off with a 2% saltwater brine (1 cup or 250ml water mixed with 5 grams salt)

Let the red onions ferment for 3-7 days at room temperature. You can test them any time after 3 days. If they aren’t fermented enough for you, put the weigh and lid back on and let them ferment longer.

Once the onions are fermented to your liking, remove the lid and fermentation weight, add a plastic lid to the jar and store in the refrigerator for up to 3 months.

fermented red onions in ramekin

Tried this recipe? Please leave a ⭐️⭐️⭐️⭐️⭐️ star rating in the recipe card below and/or a review in the comment section further down the page. We always appreciate your feedback. You can also save the recipe for later by pinning it or clicking on the heart in the lower right hand corner.

Yield: 1 pint mason jar

Fermented Red Onion Recipe

fermented red onions in ramekin

Turn raw onions into a healthy, tasty condiment to use for sandwiches, wraps, burgers, hot dogs, and avocado toast. This easy to customize recipe will show you how to ferment any onion.

Prep Time 15 minutes
Total Time 15 minutes

Ingredients

  • 1 pound (453g) sliced onions
  • 1 1/2 tsp (9g) salt

Instructions

Peel, rinse and slice the onions. I like to slice them very thin.

Put the red onion slices in a bowl and massage the salt into the slices for a few minutes. You'll notice that the onions will release their juices (sweat) and create a brine in the bowl. The onions will break down and start to soften as you massage them. When the onions no longer feel crisp, they're ready, this should take about 5 minutes.

You can add a few peppercorns or red pepper flakes to the onion slices for additional flavor, if you like.

Put the onion slices and the juice in a wide mouth pint size mason jar. Push the onion slices down into the jar. Insert a fermentation weight to keep everything under the brine and screw on a fermentation lid. If there is not enough brine from the onion juices you can top it off with a 2% saltwater brine (1 cup or 250ml water mixed with 5 grams salt)

Let the red onions ferment for 3-7 days at room temperature. You can test them any time after 3 days. If they aren’t fermented enough for you, put the weigh and lid back on and let them ferment longer.

Once the onions are fermented to your liking, remove the lid and fermentation weight, add a plastic lid to the jar and store in the refrigerator for up to 3 months.

Notes

If you want to ferment more or less than 1 pound of onion slices, you can use the instructions below to figure out how much salt you need.

Weigh the onions in grams and then multiply that number by .02 (2%) to get the number of grams of salt you will need.

Weigh the salt (different salts have different weights per volume; for example, finer salts will weigh more per tablespoon than coarse salts)

fermented red onions in ramekin

Thanks for sharing!

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