Operating within the Organizational Leaning Effect, Postmortems are an excellent way to learn something from a significant outage or other costly event. Typically, something bad happens that was unexpected indicating a miss with risk management. This is where you would normally aim to ensure this event does not happen. But alas, using a Postmortem to help fill that void is just the ticket to improve on what happened and eliminate recurrence to at least some degree.
Ideally you will want to conduct the postmortem ASAP after you have fully recovered from the event in question. Success in plugging this gap depends a great deal on honest exploration of the details that lead up to the event and must include everyone involved from realization to remediation.
There are several specific techniques from 5 Whys or other RCA techniques some of which you can find Miroverse or LucidSpark templates for.
We won’t go into more details here beyond pointing out that we do cover this in the Organizational Learning Dojo and can also do this as Postmortem as a Service both of which can be started with a free initial consultation.