Besides the master courses offered under the Mastermath framework, the mathematics departments that make up Mastermath also offer a selection of local courses which are often open to students from other departments. These local courses fall outside Mastermath's purview. All we do here is to provide pointers to where one can find information about them as they are likely to be of interest to many students following mastermath courses.
Rijksuniversiteit Groningen (click on the relevant track for you)
Universiteit van Amsterdam
Vrije Universiteit Amsterdam (search for "mathematics" in the appropriate year)
the GQT cluster keeps a list of all geometry related local courses offered in the Netherlands.
LNMB offers PhD level courses as well as the mastermath courses.
Notice that each university will have its own set of rules on the steps external students have to take to enroll in local courses. Also each university will have its own set of rules on how to get credits from courses taken elsewhere. Please check the Education and Examination Regulations of the relevant universities if you intend to attend local courses as an external student as part of your programme.