The in-service teacher training provided in the public schooling system has often been criticised on grounds of both inefficiency and ineffectiveness. Two principles that can overcome these weaknesses are a “third-space” curriculum built around problem-solving practices of teachers and leveraging the adaptability of online mechanisms for professional development. We describe a large-scale experiment based on these principles. The main lesson is that our training establishments need to develop new technical and academic capabilities, or at least partner with institutions having the necessary expertise.