Put the onion in a non-reactive vessel along with a large pinch of salt.
Pour enough vinegar on top to just cover.
Let stand, stirring once or twice, for at least 15 minutes and up to 1 hour (the onion will grow softer and more pickled-tasting as it sits).
Drain, then add to sandwiches, salads, and other dishes where raw onion would be used.
