According to Rav Elyashiv (shvus Yitzchak on sheyiha perek6:2 and Orchos Shabbos 2:in the haaroh 107) and The Chazon Ish 37:8 the entire area of the blech even the part that is NOT yad soledes bo is considered like the cover on the fire. THerefore it will be assur to do a hanacha lchatchila even on the cool side of the blech. It is based on the fact that the same way there is no chiluk for the din toich even on the cold parts , because it has a shem kirah, so too on top when you have a blech spreading the heat we can’t be mechalek two parts of the blech and the cold part is batel to the hot part. See Igros Moshe chelek 1 siman 94 where he is meilkel however in OC4 74 he is more machmir and in YD2 75 Rav moshe seems to say it is all called an oven ??”?
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