This was asked in an interview. I had one such experience in the past and at that time a single developer and PM initially conducted a pilot (manual + semi-automated) for a month/two to understand the requirements well. In other scenarios it might require the team to build an initial prototype/beta and then seek feedback from users before finalizing on the final requirements. Once you have the mvp requirements nailed out then it follows the regular process of finalizing the roadmap for the remaining 7 months or so with periodic checkpoints and finally a month for user acceptance.
But the interviewer did not look happy with that response. So, just checking with others if there is another method to handle such a requirement.
The only other thing I can think of is lean startup kind of methodology where I build iteration after iteration and move the product closer and closer to acceptance (as and when the requirements get clearer with every release). But that way I will never be sure if I can meet a fixed 9 month delivery schedule.