Combine onion and cucumbers in a clean spring-top jar. Bring water, cider vinegar, kosher salt, mustard seeds, turmeric, celery seeds and pickling spice to a boil in a large nonreactive pot or saucepan and reduce to simmer for 4 minutes.
Pour pickling liquid over the onion and cucumbers, completely filling the jar. Top off as necessary after jar cools. Refrigerate.