With more programmes being taught through English in non-Anglophone contexts, higher education lecturers are faced with new challenges. This article briefly presents a professional development initiative carried out at departmental level as an intervention for all English Medium Instruction lecturers. In order to assess the effect of such an intervention, a mix of different data sets are used, moving from self-assessment by participants to observations of individual teaching practices in the classrooms. It is shown how corroboration of surveys and observation reports in an explicit feedback template can be used to evidence the value of the intervention in context.