Fostering a culture of professionalism is the work of all stakeholders in a school and can be influenced by participating in self-selected professional development. Professionalism is a trait among teachers that has garnered significant attention within mathematics education. Central to the principle of professionalism is the recognition that educators and their peers bear some responsibility for students' mathematical success. Mathematics teachers who exemplify a culture of professionalism engage in collective professional growth by sharing what they have learned with their teaching community and holding their colleagues responsible for the implementation of research-informed practices shown to improve student learning. With a focus on providing PD opportunities that encourage teachers to think more deeply about student thinking, a group of researchers and mathematics educators developed the Online Reflection and Community-Based Instructional System Professional Development. Each of the three workshops within the PD engaged teachers in an ongoing process of (a) doing a math problem as a "student," (b) discussing their solutions, (c) looking closely at K-12 students' work for the same problem, and (d) selecting evidence within the student work to make noticing or wondering comments explaining why they chose that particular evidence. As teachers participated in this collaborative process, they articulated the benefits of engaging with and examining the problem from both the students' and teachers' perspectives. This led them to value diverse approaches to problem solving and prompted them to consider how closely looking at student work might help them provide more productive feedback to students in the future. Over the past three years, 62 K-12 math teachers, coaches, and administrators from 11 states with 3-20+ years of classroom experience have participated in this online PD. Participants shared how our PD has influenced their classroom practice and their understanding of and feelings toward professionalism.