Place sliced onions in a glass jar that is large enough for pickling liquid to fully cover onions.
Next, pour hot water over onions with enough hot water to cover onion slices. Let sit for 1 minute. This will slightly blanch the onions so that they don't have that strong onion aroma but will remain crisp. Drain and place onions back in jar.
Then place 1½ cups water, the vinegar, salt and sugar in a small saucepan over medium heat. Heat mixture until salt and sugar dissolve but not to boiling. Whisking mixture while heating.
Pour mixture over onions. Let cool and refrigerate for up to 2 weeks.