Any thickness will work, but slightly thicker slices will retain more crunch. If you have a mandolin, it can make quick work of this task.
Combine warm water, apple cider vinegar, sugar, and salt.
Pour pickling liquid over onions, making sure the onions are fully covered.
Let onions soak in liquid until they are “pickled”. 30 minutes is enough to give them light pickle flavor, but you can refrigerate them for up to a month.