Step 1

Place vinegar, sugar, salt, mustard seeds, coriander and chilli flakes in a saucepan over medium-high heat. Bring to the boil. Reduce heat to low. Simmer, uncovered, for 5 minutes.

Step 2

Peel onions and thinly slice into rings. Pack 1/2 the onion into a warm sterilised 2-cup-capacity glass jar (see Notes). Carefully pour enough hot vinegar mixture into jar to cover onion. Repeat with remaining onion and hot vinegar mixture. Seal jar. Set aside to cool. Refrigerate overnight to allow flavours to develop. Once opened, use within 2 weeks.