Slice the peeled, trimmed and halved onions to roughly 1/16″ and ensure they are completely separated and not sticking together to increase their surface area.
Combine water, vinegar, sugar and salt in a sauce pot. Whisk until the sugar and salt are dissolved and bring to a boil.
Add the onions to the boiling pickling liquid and stir to ensure they are coated and equally distributed. Allow the onions and pickling mixture to get back to a simmer, shut down and pull the pot off the heat.
Pour the pickled onions into a Cambro container and allow to cool in refrigeration.
Cover the storage container and refrigerate for 24 hours. Drain before use.