Fermented Red Onions

Full of healthful probiotics!  Use as a topping for tacos, sandwiches, wraps….        

Fermentation is a fun and easy experiment right in your own pantry. And the results are delicious and healthful.

Fermented red onions are closer in texture and taste to cooked onions than raw. The process of fermentation breaks down the onion’s crisp texture, making them milder with a delicate crunch.  And fermenting creates probiotic cultures that are healthful for our gut.


  • 2 cups non-chlorinated (or filtered) water
  • 2 tsp sea salt or kosher salt (not iodized)
  • 1 large red onion, sliced thinly
  • 2 teaspoons (or more) sugar, or other sweetener (optional)


  1. Prepare canning jars: It is not clear whether sterilization of jars is necessary for fermenting vegetables as the lactic acid bacteria create an environment that other microbes do not like. That said, better safe than sorry ….so, a painless method of sterilization can be achieved using your microwave.  Wash the jars for canning with soap and water and rinse well.  Microwave your glass jars for about 4 minutes until the water has evaporated.  Cool jars to room temperature.
  2. Add the sea salt to the water and stir to dissolve.
  3. Put the sliced red onions into a mason jar, packing them in tightly.
  4.  Pour the brine over the onions, leaving one inch between the top of the jar and the top of the brine. At this point you either need airlock lids (recommended if you are going to ferment on a regular basis) or need to place a weight on the onions to keep them submerged in the brine so that they do not mold. To do this, you can use fermentation weights (or a small zip lock bag filled with a few tablespoons of water and burped of air) to set on top and weigh-down the onions.
  5. Place the jar into a small baking pan (to catch any overflow) and cover the jars. We like to cover the jar loosely with its own lid. Other options are to use an airlock lid (find online) or cover with a cloth. You can cover and screw the lid on tightly but if you do this, you have to remember to “burp” it every day or so!
  6. Place the jar in a cool dark place (below 70F) such as a lower kitchen cabinet or pantry.  After 3 days, check for signs of life: bubbles, and clouding.  Tap the jar, looking for tiny bubbles that rise to the top. Taste.  Your fermented onions should smell oniony and taste pleasantly tangy/sour. The longer you ferment the softer and more tart the onions will become and the more beneficial bacteria will be present. Ferment up to 3 weeks. (We usually ferment for about 2 weeks).
  7. After fermentation, if desired, add 2 teaspoons sugar (or more), mixing well to combine.
  8. Refrigerate.


If more brine is needed, make additional with the same ratio- 1 heaping teaspoon (7 grams) sea salt per one cup of water, for a 3% brine solution.

If you notice a white film on the top of the jar, you may have white mold which is harmless but may change the flavor of your onions. This will not happen with an airlock lid, making these lids a worthwhile investment.