Once water has reached boil point, hold sieve with onions over the sink and pour water over them VERY CAREFULLY. This will blanche the onions and take away some of their harsh bite. Once they've drained, add to the jar with the vinegar mixture. If some of the onions are still exposed, add a little water to submerge them. Shake to mix together and then place in fridge for at least 30 minutes. In the meantime, work on the rest of the salad.