Improved pedagogical practice does not happen in a silo; it requires impetus. External influences and pressures and internal motivations are drivers for pedagogical change. Professional learning (PL) programs, in their multitude of forms, are key tools for affecting teacher change. In response to an increased focus on fostering students' reasoning in curriculum documents, our team developed the Mathematical Reasoning Professional Learning Research Program (MRPLRP) to support teacher change. This two-phased project aimed to build teachers' knowledge of the critical aspects of reasoning pedagogical approaches that foster students' development of reasoning. Phase One involved researchers' planning and demonstrating a lesson with a focus on reasoning. In Phase Two, a peer learning team (PLT) is formed to plan a lesson to elicit reasoning and to observe each other teach the lesson. This article reports on Phases One and Two of the MRPLRP from the perspective of one teacher who participated in both phases. The findings provide insights into the aspects of PL that were critical in shifting this teacher's understanding of reasoning and approaches to teaching reasoning. Whilst the results of a single case cannot be extrapolated to a larger population, we present and discuss the factors of this PL program in raising the awareness of critical aspects of reasoning, and thus this paper has the potential to impact future PL design.