Can an Ashkenazi take out a piece of boiled chicken from the fridge on Shabbos morning and place it on the plata to heat up?
Bewfore answering you and as a side point, R’ Moshe Feinstein zt”l held that even dry food cannot be placed directly on the plata, even though it doesn’t have adjustable setting. This is because it is mechzi k’mivashel, therefore it would have to be placed on an upside down pan etc.
Regarding placing even on a hot pot, it is controversial. According to some Poskim one should not warm up cooked chicken on a hot pot since the chicken will get hot from dry heat. It will get baked after being cooked. Other Poskim disagree and say that it is only an issue if the chicken will get a “roasted” taste, but not if it will merely get hot. In this instance we can rely on the lenient opinion, since the chicken in any case will not get so hot.
Rishumei Aharon 2-318-4, Am Mordechai pg. 49. This was also confirmed with Reb Dovid zt”l that Rav Moshe zt”l held it was mechzi since one could cook on it (get water to yad soledes). Regarding tzli achar bishul see M:B 318-41 and Biur Halacha, Shmiras Shabbos Khilchoso ( old edition) 1-60, Orchos Shabbos 1-29. However see Chazon Ish O:CH 37-14, Meohr Hashabbos 2-8 ftnt. 12, M’bais Levi 6 Hilchos Bishul 3, that this is also the view of the Kaf Hachayim. Also see Orchos Shabbos that it is possible that the Mishna Berura would agree in this case that it is permitted