Place sliced onions into clean sterilized jars, pack tightly. (If I'm out of mason jars, I like to use old spaghetti sauce or pickle jars! Just make sure they're well cleaned.)
Combine vinegar, water, sugar and salt in a microwave safe glass or bowl and microwave for 1 - 2 minutes, until salt and sugar dissolve when stirred.
Pour brine over onions, cover and refrigerate at least two days. When the onions have turned an even shade of pinkish purple, they're ready.