It is time to critically examine how Teaching Assistants (TAs) learn to teach and how they develop as educators. This exploration informs the design of courses to effectively foster TA professional development and deliver the best possible quality of instruction for undergraduates. Here we describe how the collaborative and multi-disciplinary Firstyear Seminar in Science (SCIE 113) educational team at the University of British Columbia (UBC), composed of graduate students and faculty members who teach the course, and a staff member who coordinates and supports the course, learn from and with each other. Our team approach creates a unique environment for TAs to grow and develop as educators, a higher level than the more common TA training. We describe multiple unique roles that TAs play in this course, showing that as contributing members of a team, they learn about teaching, curriculum development, mentorship, and leadership through observation, practice, collaboration, and reflection. The result of this approach is TAs who are well-prepared for the immediate demands of the classroom, have experienced and contributed to curricular design, have the opportunity to create and reflect on their own teaching philosophies and to apply the skills, knowledge and attitudes honed in SCIE 113 to other teaching endeavours.