I have just discovered that there is a Saint Fagan.
Unfortunately, the usually brilliant Catholic Forum Index of Saints does not include any reference to him (or her), so I have been unable to ascertain whether he/she is in fact the Patron Saint of Jewish Stereotypes.
(Although Jewish stereotypes surely need a Patron Saint, the closest the index gets to Jews is Jesuits, and the letters “ster” only bring up “sterility” – the Patron Saint against it, that is, not for it. Casilda of Toledo seems a pretty safe bet if you’re sterile.)
If anyone knows anything about Saint Fagan, please get in touch. Otherwise I shall undoubtedly be sending him/her my prayers next time I’m appearing in Fiddler on the Roof.