Prep the onions: Peel the onions, cut them in half lengthwise, and trim off the root end. Thinly slice them and transfer them to the jar.
Make the pickling liquid: Transfer the vinegar, the water, the sugar and the salt to a small pot and heat over medium heat until hot and almost bubbly. Stir to dissolve the sugar.
Pickle: Pour the pickling liquid over the sliced onions and let them cool.
Store: Cover them and transfer them to the fridge. After 2-3 hours they should be ready to eat, but they'll be even better the next day.