Mashups have become the driving force behind the development of Personal Learning Environments (PLE). Creating mashups in an ad hoc manner is, however, for end users with little or no prgramming background not an easy task. In this paper, we leverage the possibility to use Semantic Mashups (SMashups) for a scalable approach to creating mashups. We present the conceptual and technical details of PLEFExt as a flexible framework for mashup-driven end-user development of PLEs. PLEF-Ext uses the Service Mapping Description (SMD) approach to adding semantic annotations to RESTful Web services, and leverages the SMD annotations to facilitate the automatic data mediation and user-friendly creation of learning mashups.