Today's global challenges are characterised by rapid change. Adjusting to these changes, requires professionals to learn at pace, then rapidly adapt how they work. To support this need, professional development programmes have been designed as Massive Open Online Courses aimed at universal reach, large numbers, and knowledge transfer. However, focus on the scale and reach of knowledge transfer is not sufficient. Professional development programmes must extend beyond learning at scale by supporting professionals not only to learn new knowledge, but also to apply this knowledge to their work. Achieving this goal means taking into account the local context by combining knowledge learned on the programme with local knowledge, by reflecting on the role, and by planning how to adapt work practice and processes. This new focus is particularly relevant to areas of healthcare that are changing at pace. The global health challenge of antimicrobial resistance (AMR) threatens our ability to treat infections and achieve the UN's Sustainable Development Goals. Mitigating the impacts of AMR requires professionals across a wide range of roles to change how they work and there is widespread recognition of the need to provide learning opportunities for professionals to build capacity to tackle this challenge. Provoked by a funder's interest in 'developing a MOOC' to support capacity building for AMR surveillance, this paper investigates how we reconceptualise MOOCs as professional learning in rapidly changing contexts and how we combine 'learning at scale' with the need for professionals to adapt and apply knowledge. The paper presents the theoretical framework and methodological approach and offers reflections on key decisions concerning the design of 'Tackling Antimicrobial Resistance', a professional development MOOC designed as a collection of online modules aiming to support professionals to reflect on how the knowledge learned applies to their everyday practice. Although this study is contextualised within the global health challenge of AMR, these findings are likely to have applicability for the design of professional learning to support other global challenges.