This instrumental case study is interested in the development of expertise of the participating teachers as well as the pedagogy of popular music and jazz singing as socially constructed phenomena. The investigation is situated in the social constructivist understanding of learning by Vygotsky. It thus builds on development of expertise, collaboration, and conversational learning. The data was collected through a collaborative nonformal project consisting of peer-group mentoring sessions in which the participants engaged in professional conversations. The data originated from multiple sources such as interviews, face-to-face and online professional conversations, collaborative and individual reflections, an internet platform, and a researcher’s diary. The data, collected during one academic year, was analysed combining thematic analysis and qualitative content analysis in both data-driven and concept-driven ways.