1. Pour 300ml red wine vinegar or apple cider vinegar into a pan and add 3 tbsp golden caster sugar, 1 tbsp sea salt and a few black peppercorns.
  2. Bring to a simmer and cook for 1-2 minutes until the sugar and salt have dissolved. Remove from the heat.
  3. Place 3-4 sliced onions in a container and pour over the hot liquid.
  4. Allow to cool, then place in fridge to chill and leave to pickle for at least two hours before use.
  5. Serve with curries, salads, tacos or sandwiches.