Mathematics teacher educators frequently provide in-service teachers (ISTs) opportunities to practice teaching within the university classroom. While these opportunities are valuable, some question whether they focus too narrowly on learning a particular procedure for teaching instead of learning why and when to use that procedure. To address this concern, we engaged ISTs enrolled in a graduate degree program in the co-creation of practice-based lessons. This study examines what and how ISTs learned during the co-creation process. The results indicate that ISTs improved their understanding of facilitating classroom discussions in three crucial areas: asking conceptual questions, building connections, and using talk moves. Qualitative interview data revealed that ISTs learned through IST collaborations, instructor-IST interactions, and creation versus participation. Lastly, we discuss the teacher's intricate role in the co-creation process.