Peel and slice the onions into fine rings.
In your pot, combine vinegar with the remaining ingredients and heat. Let it simmer for about 1 minute until the salt and sugar are dissolved. Remove from heat.
Place the onions in the warm mixture and mix well, let them marinate for about 10-15 minutes.
Fill the sterilized preserving jar with the onions, including ginger and bay leaves, top up with the marinade – and you're done!
The onions should sit for at least 30 minutes, but they taste even better if they can marinate overnight.