- Using a mandolin or a sharp knife, thinly slice the red onion into rings.
- Place the sliced onions into a 500ml glass jar.
- In a small pot, combine the apple cider vinegar, water, salt, sugar, coriander seeds, and black peppercorns.
- Heat the mixture over a gentle simmer, stirring occasionally with a wooden spoon, until the sugar and salt have completely dissolved.
- Pour the hot vinegar mixture over the sliced onions in the jar, making sure they are fully submerged.
- Allow the onions to cool to room temperature.
- Once cooled, seal the jar with a lid and place it in the refrigerator.
- Let the onions pickle for at least a few hours, though overnight is preferable for full flavor development.
Note: The pickled onions will keep in the refrigerator for several weeks. Always use a clean utensil to remove them from the jar to help preserve their shelf life.