In warming food on Shabbos, there are three issues to consider – the first is potentially d’oraita and the latter two are d’rabbanan – bishul, hatmana and chazara.
Bishul/cooking includes, but is not limited to, finishing off the cooking/baking of a food (e.g. baking/warming a challah in a manner that removes the last vestiges of doughiness) and heating a cold liquid to above 120°F even if the liquid was cooked once before (e.g. gravy). Hatmana/insulating refers to the prohibition to wrap hot or warm foods on all sides in a manner that preserves the food’s heat. Before Shabbos, hatmana is only forbidden if the insulation causes the food to become hotter (e.g. wrapping food in extra layers of foil before placing the food into an oven which is on), but on Shabbos hatmana is forbidden even if it just maintains the food’s heat. Our discussion will focus on the third issue – chazara.
Even if food is fully cooked, hot, and not wrapped, Chazal legislated that it may not be put into the fire or onto a blech. [Right after food it taken off the fire or blech, it may immediately be returned to the blech under certain conditions, but that is a separate discussion]. Thus, on Shabbos morning, one may not take a fully cooked kugel from the refrigerator and put it onto the blech. The Poskim give two reasons for the issur of chazara. Firstly, putting food onto the fire will give the impression that one is cooking (nirah k’mivashel) and might mistakenly lead others to think that cooking on Shabbos is permitted. Secondly, if the food isn’t warming up fast enough, the person might ‘stoke the coals’ (i.e. raise the flame) (shema yechateh).
Warming Food on Shabbos for the Daytime Meal:
How then can one warm up food for the daytime seudas Shabbos? Traditionally, this was done by either putting the food next to, rather than on top of, the fire or by placing it on top of another pot filled with food (as per Biur Halacha 253:3 s.v. v’yezaher) that is sitting on the fire from before Shabbos (e.g. the cholent pot). In both of these cases, the warming is done in such an atypical manner that everyone will realize that he is warming, and not cooking, the food (reason #1), and the oddity of the situation will “remind” him not to raise the flame. In this context, food placed on a counter adjacent to the stovetop is considered “next to” the fire but food placed on the hot parts of the blech are “on top of” the fire even if the food is not directly over the flames (see Igrot Moshe O.C. IV:74 – bishul #32).
In recent years, two new methods were suggested for warming food on Shabbos – the k’deirah blech and hot plates/warming drawers, as follows.
K’deirah blech: The k’deirah blech is a rectangular, covered, shallow pan which is filled with water and placed (before Shabbos) over the stovetop. Its proponents suggest that the water in the pan renders this a “pot of food” and therefore food from the refrigerator can be warmed up on top of it on Shabbos. However, the k’deirah blech’s use in this manner has been widely condemned by Rabbanim because, as noted, the only reason one may warm up food on top of another pot of food is that doing so is so odd and atypical that it reminds people not to raise the flame and isn’t mistaken for cooking. However, the k’deirah blech is so unobtrusive and it’s “food” so inconspicuous, that people using it barely realize that it is any more than a standard blech (onto which chazara is forbidden even though it’s not the standard method of cooking), and the whole goal of using a “pot of food” is undermined.