Can one place frozen challah on top of another pot on the blech on Shabbos?
Answer:
Yes, this is fine. The challah is fully baked before Shabbos, and therefore even if the challah will harden a little, there is no issue of “cooking” it, and it can be heated on top of another pot.
See Shulchan Aruch 253:3, and Biur Halachah, who cites a dispute between Peri Megadim and Dagul Mervavah concerning placing a challah on top of the blech itself (the question being whether this can be considered mechzi kimvashel, being that bread is not baked on a blech). On top of a (full) pot it is certainly permitted.
