Apparently, generations of teachers (especially in Japan) have been using this great CPD approach, but I only found out about it several months ago, in a random article on materials writing. Here is a great resource to start with: What is Lesson Study? at Teacher Development Trust. The gist of the idea is that several teachers get together, plan one lesson, teach it in turns and meet up regularly to discuss, make modifications and reflect. What makes it different from, say, regular action research? It’s very much a team effort where several heads are put together and different strengths and weaknesses of the teachers can compensate each other (and it’s also more fun). Also, because one and the same lesson is re-taught again and again, you can address the tiniest details and get to very interesting conclusions. I suppose some products and syllabi lend themselves better to this approach, but the idea is definitely worth exploring!