We found him! We found that Jewish fictional detective! And it all has to do with a French woman named Fred.
No, the French woman named Fred is not the Jewish fictional detective. She is a writer, Fred Vargas (actually, Frederique Audouin-Rouzeau), a very popular crime novelist in France. Vargas' most famous character is Commissaire Jean-Baptiste Adamsberg.
Because all fictional detectives have to be quirky, Adamsberg is very forgetful, very disorganized, very disheveled, very promiscuous, and, (of course!) very brilliant. Alas, he is not very Jewish: from what we read, religion doesn't play a role in his life. Yet Vargas made a point to give him a Jewish surname to go with a Christian given name to make the character a bit more complex and conflicted.
So there we go, a Jewish fictional detective!
Wait a minute... Wasn't there one in Michael Chabon's "The Yiddish Policemen's Union"? Of course there was, Meyer Landsman, duh! Ummmmm...
So there we go, a French Jewish fictional detective! Hooray!