Easy Lacto-Fermented Red Onions (2024)

I was first exposed to the myriad possibilities of red onions back in my Whole30 days. There was this one recipe making the rounds on the Paleo Internet in which slices of red onion were drowned in EVOO and a little dried oregano, and then left to marinate on the counter for hours. It was legendary.

I’ve since left the Whole30 and its oil-drenched accoutrements in the past (honestly, my digestion thanks me), but the red onions remain! You’ve probably seen recipes for pickled red onions, which I love- but I favor these Lacto-Fermented Red Onions, both for their tangy flavor and probiotic goodness!

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Why You’ll Love This Recipe

These onions add color and flavor to everything, from salads, to tacos, to burgers and other sandwiches. The possibilities are endless. I have even been known to eat them straight from the jar. And no, I’m not insane enough to bite an onion like an apple: the fermentation process makes the onion flavor much less offensive.

These are also an incredibly easy fermented food to make-most likely, you’ve already got everything you need! They do require some patience, as the fermentation time is usually about 5-7 days.

Pickled vs. Lacto-Fermented Red Onions: What’s The Difference?

I absolutely adore the pickled onions recipe featured in this photo. I used this one from Pinch of Yum. It’s a classic quick-pickling situation, in which the onions are submerged in a vinegar/water solution for a few hours.

Quick pickling is a way to speedily “pickle” and preserve a vegetable, without the added steps of canning for long term storage. Pickled vegetables, including pickled red onions, taste, well, pickled. There’s a notable vinegar flavor to them, though other spices and flavorings can be added as well. Pickled vegetables are often brightly and beautifully colored, like the onions above.

Lacto-fermentation, on the other hand, relies on a salt & water brine to promote the growth of lactobacillus bacteria. The lacto-fermentation process is an ancient method for preserving food- and the bacteria are terrific for your gut health! More on this here.

The lacto-fermentation process results in a tangy taste that will be familiar to those who’ve practiced lacto-fermentation of vegetables before–it even reminds me a little of kombucha! The lacto-fermented red onions definitely lack the vinegary taste that the pickled ones have, but they’re otherwise somewhat similar. Both onions taste milder and more nuanced than they would when eaten raw!

Tools & Equipment for Lacto-Fermented Red Onions

• Jar: I prefer a wide-mouth mason jar
• Large liquid measuring cup
• Kitchen scale
• Fermentation weights (optional but handy!)

Ingredients for Lacto-Fermented Red Onions

• Red onions
• Sea salt, or kosher salt: avoid table salt-no need for added iodine
• Whey: an optional, but very effective, starter culture. If you don’t have any, you’ll simply use more salt!
• Filtered water
• Sugar

The Process: How to make Lacto-Fermented Red Onions

Prep

Sterilize your jar: I do this by placing my jar in a clean sink and pouring boiling water over it. Alternatively, fill the sink with boiling water and submerge the jar. Remove the jar to a clean area on your counter (use tongs or a silicone oven mitt!) and allow it to cool before using it.

Peel and slice your onions.

Make The Brine

Prep the brine: I haven’t included amounts in this recipe, because I like to do it by weight! I recommend making the brine with 3% salt by weight if you ARE using whey, or 5% salt by weight if you are NOT using whey. Whey is an effective starter culture, but when you’re fermenting vegetables, it can be omitted and replaced by more salt.

To mix your brine, weigh about 2 cups of water, in grams. Calculate your salt: amount of water (in g) x .03 (or .05) = grams of salt. Add your calculated weight of salt to your water, and mix to dissolve salt.

Pack your sliced onions into your prepared jar. Add a generous pinch of sugar and whey, if using.

Pour brine over your onions. Make sure the brine fully covers the onions, to avoid mold. Fermentation weights can help ensure that the onions stay submerged!

Cover your jar. If you use the regular lid, make sure to remove it once a day while the jar is on the counter, to release any built up gas from fermentation. There are special lids out there that are designed to release these gases for you, but I’ve yet to invest in these: it’s easy enough to burp the jar daily! Also, I haven’t found red onions to produce a huge amount of excess gas during fermentation, but don’t risk it.

Ferment!

Leave the jar on the counter at room temp to ferment. Fermentation time varies by room temperature, but will likely take about 4-7 days. Begin tasting after four days, if you are seeing some signs of fermentation, like bubbles appearing in the jar.

When fermentation is complete (meaning when the onions taste right to you), remove weights, tightly cap jar and move to the fridge.

How Should I Serve & Store Lacto-Fermented Red Onions?

Store in the fridge! These should keep in the fridge for weeks to months.

As I’ve noted above, you can definitely just eat these straight from the jar, if you’re so inclined. But they really shine as a topping on all sorts of dishes: tacos, a burger, a sandwich, a salad, gyros…really, is there anything that onions don’t go well on? Probably, but I’m not sure what.

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Did you make this recipe? Let me know how it went in the comments! If you share on Instagram, please tag me @sisuhomemaker!

What Should I Make Next?

Have you branched into fermented dairy yet? Info on Finnish yogurt (viili) here!

Or check out .