Many elementary teachers find the complexity of understanding and teaching mathematical reasoning challenging. Teachers can benefit from professional learning (PL) programs designed to develop strategies to identify reasoning and implement it in mathematics lessons. This paper reports on a PL program designed to support a Peer Learning Team (PLT) of elementary teachers in Canada who were assisted by a researcher to peer-plan, peer-observe, and reflect on lessons fostering reasoning. Recorded data from PLT meetings were analysed against a planning framework to study the teachers' growth in understanding reasoning. The findings revealed teachers engaged in a PLT which plans together to embed reasoning in a lesson, followed by peer-observation and reflection is a powerful and effective model of PL for understanding mathematical reasoning and pedagogical approaches that foster students' reasoning.