Mentoring can support beginning teachers with various challenges of their career start. At the same time, many beginning teachers have no or insufficient access to high-quality mentoring opportunities. The NEST (Novice Educator Support and Training) project implements a mentoring program in seven European education systems aimed at teachers in socially disadvantaged schools, seeking to ensure support by creating adaptivity to different school contexts and, in particular, to the needs of mentored teachers. In this article, we provide insight into the evaluation concept for the mentor training that is central to the project. Furthermore, we show how the project deals with the challenge of developing methods for an evaluation in the contexts of different educational systems together with local partners, which considers the perspectives of both mentors and teachers. We present the evaluation design and results on changing and assessing the practices of mentors from three contrasting education systems. Lastly, we discuss consequences for the project and its ongoing evaluation.