As for now, Hangfire deals only with synchronous method invocations. You can not use the await keyword inside your background methods. You should wrap your async method into another method to call it synchronously – http://stackoverflow.com/questions/9343594/how-to-call-asynchronous-method-from-synchronous-method-in-c.
Thanks for this question, I’ll make background job client to throw an exception for this case and think how to deal with async methods later. With correct implementation they can help to improve the processing throughput for I/O-bound background jobs.