I have a web application that requests a report that takes more than 10 minutes to run. Apart from improving that performance, I would for now prefer to set up a thread to run the report and mail it to the user, returning that decision message back to the user immediately.
I have been looking at cherrypy.process.plugins.Monitor, but I'm not clear if it is the correct choice (what to do with the frequency parameter?)
Monitor is not the correct choice; it's for running the same task repeatedly on a schedule. You're probably better off just calling threading.Thread(target=run_report).start(). You can then return 202 Accepted to the user, along with a URL for the client to watch the status and/or retrieve the newly-created report resource when it's ready.
The one caveat to that is that you might want your new thread to shut down gracefully when the cherrypy.engine stops. Have a look at the various plugins for examples of how to hook into the 'stop' channel on the bus. The other option would be to make your thread daemonic, if you don't care if it terminates abnormally.