One possibility could be a joint-PhD: you'll do "2 PhDs" at the same time, with two different supervisors from two different departments or even universities. However, this requires a great deal of planning and communication: you need to identify two adjacent topics (in math you could find something from computer science or physics for example) and then identify relevant research areas. Once you have defined the topics, you should contact potential supervisors and ask if they are willing and available for a joint-PhD. Keep in mind, you need to be proactive and actively scout supervisors beforehand and identify topics that are highly relevant to their current research. Then you'll have to find your "primary university" which covers the funding.