The best way to determine corrective actions after a major information security incident is to conduct a postmortem assessment, which is a systematic and structured review of the incident, its causes, its impacts, and its lessons learned. A postmortem assessment can help to identify the root causes of the incident, the strengths and weaknesses of the incident response process, the gaps and deficiencies in the security controls, and the opportunities for improvement and remediation. A postmortem assessment can also help to document the recommendations and action plans for preventing or minimizing the recurrence of similar incidents in the future.
References = CISM Review Manual, 16th Edition eBook1, Chapter 4: Information Security Incident Management, Section: Incident Response, Subsection: Postincident Activities, Page 211.