The Real Source Of The Shlissel Challah Phenomenon
The original source for the Shlissel Challah Segulah is from the Sefer Ohev Yisroel who claimed Shlissel Challah to be an old Minhag B’Yisroel .
The Sefer Ohev Yisroel is a collection of Divrei Torah from Reb Avrohom Yehoshua Heshil zt”l, known as the Apte Rov (Rebbe).
The Rebbe's son R. Yitzchok Mier zt”l writes in the Hakdama:
"Many people were quoting inaccurately Toirehs from the Rebbe and thereby saying over in his name things he never said, Therefore they appointed a Talmid Chacham to write down all the authentic and reliable Divrei Torah of the Rebbe.
The Kesovim were handed in to the Rebbe who at times corrected them and chose the Toirehs he agreed with."
In the year 1863 long after the Petira of the Rebbe and his son Reb Yitzchok Mier, the grandson R. Meshulem Zusia zt”l finally printed the Sefer Ohev Yisroel.
He also added to the back of the Sefer, “Likutim”, Divrei Torah that wasn’t recorded in the Sefer Ohev Yisroel.
There are two possibilities, why it didn’t make it in the Sefer.
a) The son R. Yitzchok Mier and the Talmid Chacham were not aware of that particular Dvar Torah.
b) The Rebbe never said it. It was made up by one of the many people who claimed
the Rebbe to have said it. It's also possible the Rebbe himself to have discarded it as mentioned in the Hakdama the Rebbe chose the Toirehs to be entered.
R. Meshulem Zusia zt”l claimed to believe all the Toirehs to be of his grandfather being he heard them from reliable people (not as reliable as the original Sefer which was
edited by the Rebbe himself) and therefore added them to the back of the Sefer.
Others* claim that Shlissel Challah is rooted in Christian
and/or pagan practices.
Keys used to be manufactured in the form of a cross, and at Easter time, Christians would bake them into a rising loaf of bread to symbolize Yoshke rising from the dead.
(This is the source of the "hot cross buns.")
The Chida (Moreh B'etzba 7:190) writes " This is a Klall Gadol.
You should not** follow any Minhag unless you know the root (source)
of it from Chazal , or Authentic Kisvei Arizal or you know the true
reason behind the Minhag".
* Their opinion may be considered only on the assumiption the Rebbe never mentioned this Minhag
** He is probably referring when introducing new Minhagim into
the family or Kehila.. Family or Kehila. Minhagim from the past needs no source or reason (אמבצי).
(Ashkenaz families and even many Chasidishe families never practiced this Minhag 50 years ago.)