Developing knowledge and practice for high-quality K-12 class discussion remains challenging, especially for new teachers juggling other classroom responsibilities. Our study reports the case of a preservice teacher learning to lead discussions while enrolled in a teacher education inquiry course, simultaneous with semester-long supervised practice teaching in a seventh-grade class (12-13-year-olds) in a high-poverty urban community. The work is guided by a complex teacher learning process for developing complex practice of facilitating discussions in culturally and linguistically diverse high school English classes. Countering popular approaches to "talk moves" as useful but often generic facilitation practices, the teacher education pedagogical innovation we describe positions teachers as knowledge-generating, agentive professionals. Our conceptual framework for teacher learning features dialogic teacher inquiry, with three domains. The first domain involves moving beyond methods texts to dialoguing analytically with and among multiple print, online, and mentor resources for supporting development of a dialogic teaching stance. The second domain intentionally guides new teachers to explore classroom data and consider students as knowledge resources in shaping instruction. The third domain sustains dialogue about discussion processes and evolving conceptions of dialogism in small groups of preservice teacher collectives, enabling sharing of inquiry data, emerging findings, and dilemmas of practice. Drawing upon a larger database, we present a case study demonstrating one preservice teacher's inquiry work with deep analysis of student talk, detailed memoing processes featuring challenges and benefits of developing dialogic teaching practices, thoughtful criticism of long-established discussion practices, and discoveries about nuances of dialogic teaching. Our case contributes to the literature by presenting an example of dialogic pedagogy for teacher education, in service of preservice teachers learning to lead classroom discussions. Additional innovative pedagogical designs are needed to assist teachers in gaining complex knowledge and practice for teaching and promoting meaningful and learning-rich talk in K-12 classrooms.