Bring a pot of water to a boil. Whisk the vinegar, salt, and sugar in a bowl until combined, then add it to whatever container you will use to store the onions (mason jar works well). Place the onions in a sieve or colander in the sink. Slowly pour the boiling water over the onions. Put the blanched onions in the container with the pickling liquid and mix it all up. Close until ready to use