This paper investigates in-service intermediate phase mathematics teachers' metacognitive awareness based on their engagement in a professional development intervention in two rural and one township school in South Africa. Professional teacher development interventions should be structured and collaborative as teachers tend to revert to traditional modes of teaching when interventions are brief and sporadic. This research was scaffolded across the zone of proximal teacher development (ZPTD) as conceptualized by Warford (2011). The aim of the study was to investigate how metacognitive awareness facilitated intermediate phase mathematics teachers' ZPTD, i.e., from their actual knowledge to "proximal development". 10 intermediate phase mathematics teachers participated in the qualitative research (n=10). The professional development intervention was conducted on two consecutive Saturdays, infused with an adapted lesson study approach. The data was collected through (i) individual interviews, (ii) reflective prompts, (iii) observations, and (iv) reflective journals. The data from each data source were coded and categorized and were triangulated to answer the research question. Participants deemed the professional development intervention and the accompanying materials as supportive and showed increased metacognitive awareness of their personal teaching praxis during and after the intervention, hence growth along the ZPTD occurred.