Software developers who want to start contributing to an Open Source Software (OSS) project often struggle to find appropriate first tasks. The voluntary, self-organizing distribution of decentralized labor and the distinct nature of some OSS projects intensifies this challenge. Mentors, who work closely with newcomers, develop strategies to recommend tasks. However, to date neither the challenges mentors face in recommending tasks nor their strategies have been formally documented or studied. In this paper, we interviewed mentors of well-established OSS projects (n=10) and qualitatively analyzed their answers to identify both challenges and strategies related to recommending tasks for newcomers. Then, we employed a survey (n=30) to map the strategies to challenges and collect additional strategies. Our study identified 7 challenges and 13 strategies related to task recommendation. Strategies such as "tagging the issues based on difficulty,""adding documentation,""assigning a small task first and then challenge the newcomers with bigger tasks,"and "dividing tasks into smaller pieces"were frequently mentioned as ways to overcome multiple challenges. Our results provide insights for mentors about the strategies OSS communities can use to guide their mentors and for tool builders who design automated support for task assignment.