I've been wondering lately about the Jewish Temple at Onias. This is all I know about it:
1. It was destroyed just after the Second Temple.
2. Josephus mentions it twice, somewhat contradictorily.
3. Talmud discusses the validity/sin of offerings made there, and that their priests cannot officiate in Jerusalem, but can partake of the priestly portion etc. (also that it's in Alexandria...)
What else can I learn? Is there any evidence to suggest its practices were different from Jerusalem? In the Hebrew Bible there is a lot of motivation/historiography about the building of the First and Second Temples. But do we have a justification for the building at Onias? Like a postBiblical document or a secular source? (like the Elephantine scrolls) Why does the Talmud consider Onias valid? And last, is there any archeological evidence, like on Elephantine?
EDIT: I corrected point 3.
I can only provide information based on my knowledge of Talmud so hopefully it doesn't break the rules here. You might be interested in asking the question in r/Judaism.
Basically the temple at Onias was not considered legitimate for proper sacrifice by mainstream Jewish sources. The temple in Jerusalem is considered the only legitimate temple (where sacrifices can be preformed) by mainstream Judaism. In the Talmud they look down upon the sacrifices at Onias, but are kind of okay with it as a synagogue. However, the Jews in Egypt viewed the sacrifices as legitimate. Jewish sources reconcile this by basically characterizing their illegitimate sacrifices as an expression of their desire to be able to sacrifice in Jerusalem. The Talmud is very brief on the entire affair however.
Edit: Talmudic discussions are in Masechet Avoda Zarah page 52:B.