Traditional teacher professional development (PD) programs frequently lack sufficient face-to-face preparation time to adequately support the implementation of evidence-based pedagogical strategies in classrooms, with teachers citing insufficient preparedness and training time for new methods. To investigate potentials for online PD methods to effectively address these issues, while increasing sustainability, this study examined teachers in Title-I elementary schools spanning multiple school districts participating in a large-scale online PD certificate program preparing teachers over 10 instructional weeks to implement Next Generation Science Standards (NGSS) (S)cience, (T)echnology, (E)ngineering, (A)rts, (M)ath (STEAM) curricular instruction with goals of increasing teachers' science content knowledge, self-efficacy to implement STEAM instruction, and STEAM teaching perceptions. Paired sample t-tests indicate teachers' life science content knowledge, self-efficacy, and STEAM perceptions increased significantly after completing the online PD. Multiple regression analyses identified teachers' science content knowledge and STEAM perceptions as statistically significant predictors of teachers' self-efficacy to implement STEAM instructional strategies. Teachers' posttest course evaluations were gleaned, highlighting three emergent themes: learning experience design, modality, and online teacher collaboration. Utilizing a mixed method approach, these qualitative insights corroborated the quantitative trends, describing how specific aspects of the pedagogical learning experience design (LXD) framework operationalized in the online course fostered a practitioner development learning environment, focused on supporting teachers' learning needs. Results suggest practical design implications for sustainably increasing the efficacy of PD through online teacher training developed in conjunction with LXD models for iterative improvement and design of high-quality online instructional PD programs.