How can you get a large, legit Persian Empire after the rise of Islam?
Legit Persian meaning the ruling dynasty and ruling class have fully assimilated to Persian and have no traces of their origins--or even better, they were Persian in the first place.
Large meaning it has at least Persia, Mesopotamia, and either the Indus or the Levant.
Bonus points if it's Shi'a like OTL. More bonus points if it's Zoroastrian (far fetched, I know).
Bonus points if Mesopotamia becomes linguistically Persian.