Doctoral teaching assistants (TAs) provide key support for learning in STEM fields because they are present during exercises, labs and projects when students are actively engaging with course material. While some institutions provide training for TAs, their effect on teaching activities is rarely assessed. We use the lens of Social Cognitive Theory (SCT) to analyse data on the pre and post course teaching priorities of 20 doctoral TAs who followed a 5 day practice-intensive course on STEM HE. Course time was split between instructors modelling interactive teaching strategies to engage TAs in a data-driven reevaluation of their beliefs about teaching and having each TA teach a lesson everyday using a structured feedback loop to promote reflection. TAs reported self efficacy gains for designing instruction, addressing disruptive behaviour and managing student attention spans after the course. Their priorities also appear to shift away from ‘teaching’ and towards ‘learning’. TAs’ affective reactions and utility judgements after the course indicated that they thought the course was useful and they intended to use the strategies that they had learnt. This practice and reflection intensive course model, able to accommodate up to 40 TAs, is relevant for institutions seeking to improve the quality of undergraduate education or doctoral candidates' preparation for academic roles.