The unforeseen school closures in response to COVID-19 have brought unique challenges to teachers, who were required to not only flexibly shift between on- and off-line learning but also safely promote student-centered and collaborative learning in socially distanced remote and on-campus classrooms. Teachers in South Korea, one of the first countries to transition to blended teaching, faced similar challenges with limited time, resources, and training to prepare for this unprecedented situation. This case study provides evidence-based insights on how a teacher-initiated Professional Development (PD) session on blended teaching can support teachers to address these challenges. The PD session was designed in partnership with a Korean district office and the subject-based Communities of Practice (CoP) to provide insights on teacher-initiated bottom-up transformation in blended teaching. Both quantitative and qualitative data showed teachers' increased understanding of Technological Pedagogical Content Knowledge (TPACK) and blended teaching competencies (BTC) after the PD session. This case study informs how educators, researchers, and administrators can collaborate to overcome educational exigency. When the results are generalized across different cultural contexts, it will provide ideas and strategies for the design and implementation of intensive teacher-initiated PDs to support the flexible design and implementation of blended teaching in variable learning contexts.