Your challange, should you choose to accept it, is to find a plausible POD to create a Jewish kingdom, or make an existing kingdom having a Jewish king/strong Jewish nobility, in Europe during the middle ages. specifically, NOT the Khazars or any of the steppe tribes, I mean western Europe.
Is this at all possible, or ASB? the closest I can think of would be somewhere in Spain, but im not too familier with the time period.