Fermented Onions
Fermented onions are mildly tangy, naturally sweet, and packed with probiotics that support gut health. This simple ferment transforms ordinary onions into a flavourful condiment — perfect on salads, sandwiches, or to brighten cooked meals. Their gentle acidity and crunch make them an instant kitchen favourite.
Ingredients
For a 1 L fermentation jar.
- 650 g onions, thinly sliced
- 20 g sea salt (1 tbsp)
- 300 ml filtered water (1½ cups)
Equipment
- 1 L glass fermentation jar
- Glass weight
- Grommet lid and airlock
Method
Prepare the onions
Peel and slice the onions thinly. In a bowl, mix the slices with the sea salt and let them rest for 1 hour, massaging gently to help release their natural water.
Sterilise the jar
Clean your 1 L fermentation jar with hot water and let it air dry completely before use.
Pack and add brine
Transfer the onions along with the water they've released into the jar, packing tightly to remove air pockets. Place a glass weight on top to keep them submerged. Pour in the filtered water to cover the onions completely, leaving at least 2 inches of headspace for the gases to expand during fermentation.
Seal with an airlock
Close the jar with a grommet lid. Fill the airlock halfway with water so gases can escape while keeping contaminants out.
Ferment
Let the jar sit at room temperature (around 25–30°C) for 5–7 days.
Check daily
Press down gently each day to keep the onions submerged. Begin tasting from Day 5 — they should develop a light sourness with a pleasant crunch.
Store and enjoy
Once the flavour is to your liking, remove the glass weight and airlock, seal with a storage lid, and refrigerate. They keep well for up to 6 months.
Watch the walkthrough
Notes & variations
- Salt ratio: a 3% brine keeps onions crisp and safe for fermentation.
- Headspace: maintain 2 inches to avoid overflowing brine.
- Airlock lid: reduces the need to open the jar during fermentation.
- Flavour boost: add chilli flakes, black pepper, or thyme for a spiced variation.
- Storage: keep refrigerated after fermentation; flavour deepens over time.
Fermented onions are the perfect mix of tangy and sweet — a probiotic condiment that elevates any meal. Try them in sandwiches, tacos, or grain bowls for an instant flavour upgrade.
