TL;DR: The Meta-Retrospective
A meta-retrospective is an excellent exercise to foster collaboration within the extended team, create a shared understanding of the big picture, and immediately create valuable action-items. It comprises of the team members of one or several product teams—or a representative from those—and stakeholders. Participants from the stakeholder side are people from the business as well as customers.
Meta-retrospectives are useful both as a regular event, say once a quarter, or after achieving a particular milestone, for example, a specific release of the product. Read more on how to organize such a meta-retrospective.