Fermented Onions

Heaven is the combination of fermented onions and rosemary. Fermented onions are an easy, brine method way to preserve a plethora of onions. If you’ve ever craved a zesty kick to elevate your dishes, look no further. 

Let’s discuss what fermented onions taste like, their health benefits, uses, tips & tricks, the difference between pickled and fermented onions and more!

Fermented onions softened in a brine after several days of fermentation

What you’ll love about this recipe:

  • VERSATILE – These onions go with anything!
  • HEALTHY – Probiotic rich, greater bio-actives and antioxidants than raw onions.
  • INEXPENSIVE – Talk about an easy, inexpensive veggie…the humble onion is the one!

What do fermented onions taste like?

Fermented onions have a unique flavor profile that differs from fresh onions. The fermentation process transforms the taste and texture of onions, imparting a tangy, slightly sour flavor with hints of sweetness. It also mellows the sharpness. 

The texture of fermented onions can vary depending on the fermentation method and duration, but they typically become softer and more tender compared to their raw counterparts. 

With the addition of rosemary the onions have a nice umami, herbaceous flavor that is irresistible. And bonus, Rosemary has significant antimicrobial, anti-inflammatory, anti-oxidant, anti-apoptotic, anti-tumorigenic, antinociceptive (related to pain), and neuroprotective properties.  

How do you ferment onions?

It’s really simple, and follows the guidelines for other fermented vegetables. All you need is water, salt, onions and spices, if desired. No need for starter cultures

Thinly slice the onions, I like a mandoline. 

Layer the salt (spices if using) and onions as you place them in your jar of choice. 

Cover with water, being sure to submerge the onions completely so as not to expose to air. Lactobacillus is an anaerobic bacteria, which means it does its thing without oxygen. 

That’s it! Nothing to be scared of. 

Are fermented onions good for you?

Yes! Fermented onions can offer several health benefits due to the fermentation process, which enhances their nutritional profile and makes certain nutrients more bioavailable. 

  1. Increased bioavailability of nutrients. Raw onions are a healthful food, but fermentation seems to increase the bioavailability of bioactive compounds including phenolic compounds, flavonoids, and SCFAs, and the consumption of fermented foods increased microbiome diversity and decreased inflammation
  1. Increased antioxidants. Fermentation raised the quercetin content in onions, and subsequently increased the antioxidative and neuroprotective activities.
  1. Homemade ferments result in a greater variety of probiotics which improves our immune system. One study found that lactic acid bacteria (Lactobacillus zymae, L. malefermentans, L. plantarum), acetic acid bacteria (Acetobacter pasteurianus, A. orientalis), citric acid bacteria (Citrobacter sp., C. freundii), and yeasts (Candida humilis, Kazachstania exigua, Saccharomyces boulardii) were the dominant microorganisms involved in onion fermentation.

In a nutshell; fermented onions are easier to digest, contain probiotics, more bioactive compounds, additional antioxidants and perhaps fewer fodmaps to reduce symptoms in those with IBD. Fermented onions for the win! 

Fermented onions in a jar with fresh rosemary submerged in a brine

Tips & Techniques to a successful fermented onion

  • Size matters: smaller pieces create more surface area which speeds up the fermentation process. Choose your favorite cutting technique!
  • Err on the side of less salt: When you are making your ferment always start with less salt. It’s easy to add more after taste testing a few days in, but you can’t remove salt! 
  • Keep your onions submerged! The key to safe fermentation is to keep your ferments submerged under their brine with a fermentation weight. An easy way to do this is with a glass weight, bag filled with brine, clean rock or scrap veggie like a cabbage leaf.
  • Check the pH: a simple fool proof way to ensure the safety of your ferment is to consume once the pH reaches less than 4.6.

How do I know if my fermentation was successful?

If you like the way the onion tastes, it’s probably perfect. 

Thankfully, it’s easy to spot mold. Proper hygiene practices will eliminate salmonella and e.coli. And botulinum toxin, though it can’t easily be seen, can give signs like spurting liquid or off smells. 

To keep your ferment safe don’t ferment in temperatures over 72 degrees, and keep out of direct sunlight. Also, always start with clean tools and properly processed foods. 

If you feel better with numbers, use a pH strip and shoot for <4.6.  And check out my blog about the dangers of lacto-fermentation.

How long can you ferment onions?

The duration for fermenting onions can vary depending on personal preference and desired flavor, but generally onions can be fermented for a few days to several weeks. 

Here’s a basic timeline for fermenting onions:

  • Initial fermentation: Onions are typically submerged in a brine solution (water and salt) along with optional seasonings like herbs or spices. The fermentation process begins immediately, you may see bubble action!
  • Taste testing: After a few days, you can start tasting the onions to monitor their flavor development. You may enjoy lightly fermented onions, which may take around 3 to 5 days. Or a stronger, more complex flavor, which takes a week or more.
  • Fermentation period: Continue fermenting the onions until they reach your desired level of flavor and tanginess. This can take anywhere from 1 to 4 weeks, and varies based on preference, amount of salt and your room temperature.
  • Storage: Once the onions have fermented to your liking, transfer them to your refrigerator. Fermented onions will continue to mature slowly in the fridge, so their flavor may continue to develop over time. Monitor and use within a few weeks.

Fermented onions vs pickled onions

Pickled onions and fermented onions are both preserved in a brine solution, but they undergo different processes and have distinct flavors and textures:

  • Pickled Onions:
    • Pickling typically involves preserving vegetables in a vinegar-based brine, which may also contain sugar and spices for flavoring.
    • Pickled onions are usually made by soaking sliced or whole onions in a vinegar solution for a relatively short period, often just a few hours to a few days.
    • The vinegar in pickled onions provides a tangy and acidic flavor, while any added sugar can contribute sweetness.
    • Pickled onions have a bright, acidic taste.
  • Fermented Onions:
    • Fermentation involves the natural process of beneficial bacteria breaking down sugars in the vegetables, producing lactic acid and other compounds that preserve the food.
    • Fermented onions are made by submerging onions in a saltwater brine without vinegar and allowing them to ferment over a longer period.
    • Fermented onions develop a tangy, slightly sour flavor as the fermentation progresses, with hints of sweetness and umami.
    • The texture of fermented onions becomes softer and more tender compared to raw onions, and they may have a more complex flavor profile due to the fermentation process.

What do you eat with fermented onions?

  • Sandwiches and Burgers
  • Salads
  • Tacos and Wraps
  • Charcuterie and Cheese Boards
  • Grain Bowls and Buddha Bowls
  • Soups and Stews
  • Pizzas and Flatbreads
  • Egg Dishes
  • Dice into tuna, sardine or chicken salad

The sky’s the limit!  🙂 Now, let’s get into the recipe.

Overhead fermented onions, with a fresh cut onion and a pinch pot of salt

Fermented Onions

Experience tangy, complex flavor with fermented onions. Plus the added benefit of probiotics, greater nutrient bioavailability and more!
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Course: Salad, Side Dish
Cuisine: American
Keyword: Grab & Go, Make Ahead, Summertime
Allergen: Dairy Free, Gluten Free, Vegan
Prep Time: 10 minutes
Cook Time: 0 minutes
Fermentation Time: 4 days
Servings: 4 servings
Calories: 71kcal


  • Measuring Scoops & Spoons
  • Chefs Knife or mandolin
  • Fermenting jar
  • Fermenting weight


  • 1 cup onion, sliced, 1 medium to large onion
  • 1 cup water, distilled or similar
  • 1 tbsp unrefined salt, like Redmond's Real Salt
  • 1 tbsp rosemary, fresh


  • Thinly slice the onion using a chef's knife or mandolin.
  • With a clean canning jar (or similar) layer the onion with the salt and fresh rosemary.
  • Cover with water and submerge the onions with a weight, or onion peel if desired.
  • Allow to ferment for several days. Monitor daily. After day 3 or 4 begin tasting. Transfer to the fridge once you like the flavor and the pH is less than 4.6.


  • Option to use other herbs or spices such as oregano, basil or thyme would be great pairings. Fresh is always best, but reduce the amount by about 1/3 if using dried spices.
  • A pH of less than 4.6 reduces risk of botulinum toxin.


Calories: 71kcal | Carbohydrates: 16g | Protein: 2g | Fat: 0.5g | Saturated Fat: 0.2g | Polyunsaturated Fat: 0.1g | Monounsaturated Fat: 0.1g | Sodium: 6996mg | Potassium: 254mg | Fiber: 4g | Sugar: 7g | Vitamin A: 66IU | Vitamin C: 13mg | Calcium: 74mg | Iron: 1mg

Note: You can choose to cut the onion into bite sized pieces or larger chunks. Just a note that the fermentation time with vary. Larger pieces will require more time, whereas smaller pieces will require less time, due to the greater surface area for the “bugs” to ferment.

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