Capture lessons at project milestones, not just at the end. And make them accessible - a lessons learned document nobody can find is the same as no lessons at all.
Strong answers describe a systematic approach: structured project reviews at milestones and completion, documenting lessons in an accessible format, sharing across teams, incorporating lessons into templates and checklists, and referencing past lessons during new project planning. Best candidates discuss the cultural challenge of making lessons learned a habit rather than a checkbox.
Tests learning culture and organisational thinking. PMs who do not capture lessons repeat mistakes. Those who capture but do not apply them are going through the motions. Ask: "Give me an example of a lesson from one project that directly improved another."