Here’s my theory about the origin of this minhag. My point here is not really about advocating for the minhag. I just feel a need to push back when people belittle another community’s practices.

Those who try to say the practice was originally pagan raise Roman era practices or Irish customs involving a cross (not a key), neither of which would Chassidim in 18th century Easter Europe be influenced by. It’s fishing for an objection.

I can see problems with those who treat Schlissel Challah as a way to get Hashem to provide wealth, whether I deserve it or it is what is what is appropriate for my life story or not. But let’s assume the point of the minhag is really more like the manei milsa of Rosh haShanah. We dip an apple in honey on Rosh haShanah to aid our praying for a “good and sweet year”, so why not have a key challah as an aid to our praying for wealth?

So now, on to the theory…

The week after Pesach, it’s hard to obtain yeast. Sourdough was gotten rid of before the holiday. Fleischman didn’t invent a way to make dry active yeast yet. You had to get rid of all your starter dough.

Easiest thing to do is skim the yeast off of beer. (The term for this yeast layer is “barm”, for what it’s worth.) Problem is, the resulting yeast is way too active for normal textured bread. So, the usual thing to do was to put some metal into the dough, as metal slows down yeast. Likely a key — right size, handy. Putting a small piece of metal in order to use beer yeast to make bread was indeed done by other cultures, such as in Ireland. One doesn’t need to claim foreign influence, it’s simply the biochemistry that leads to the idea of baking a key into challah.

Now this goes on for years, maybe centuries, until women baking bread think that putting a key into the challah dough the week after Pesach is just something Jews do. (Like avoiding zugos by not boiling an even number of eggs.)

In the 18^th or early 19^th century the Apter Rav (the original R’ Avraham Yehoshua Heschel) thought about the practice. He invoked the Baal Shem Tov’s principle that no practice becomes a norm among Jews without Hashem intending us to inculcate Torah from it, he looks for a meaning.

The Apter Rav takes note of the fact that farming begins in all earnestness in the spring, and that after Pesach farmers can get caught up on their work. A perfect time to remember (as we learn in the beginning of Mesechtes Taanis) that the key of rainfall and of prosperity is always directly in the “Hand” of Hashem Himself.

And so a common practice gets promoted to a minhag, now that it has a Torah meaning. Or meanings:

1. Based on “Pischi li achosi, ra’ayasi…” …, on which the medrash quotes, “Pischu li pesach kechudo shel machat… – (roughly:) Open your hearts [in teshuvah] like the eye of the needle, and I [Hashem] will open the rest like the entry to a great hall.”
2. According to Qabbalah, Shaarei Shamayim are open on Pesach. The lower gates close after the holiday, so it is up to us to open them again.
3. In the Midbar, the Jewish people ate mon until after the first Pesach in Eretz Yisrael (with the bringing of the Qorban Omer; see Yehoshua 5:11). That’s when we transitioned to eating our own crops. The Schlissel Challah after Pesach is a request the God should open the Sha’arei Parnasah…
   Alternatively, the mon began to fall and we started to eat it in the month of Iyyar, and this Shabbat is always Shabbat Mevarchim Iyyar.

Challah baked in the shape of a key is a much more recent innovation. Apparently younger than I am. I don’t think the origin story has to account for a development that late.

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So much for theory. Some thoughts about it:

Each point has significant evidence, but if you combine a number of things that each are only probably true, I would not swear the result is.

And there are other beloved minhagim I can point to that became practices first and only acquired meaning after the fact. Some of which — like Purim costumes or eating milchigs on Shavuos — appear to have non-Jewish origins.

So it would seem to me the claim that origin of schlissel challah is an imitation of another religion’s practice is still improbable, as I explained at the top of the post. But even if it is true, we have precedent for saying that once the connection to the other religion is lost to time, finding a religious message in a common Jewish practice can be enough to sanctify a former borrowing into a full minhag.