Based on a Naaleh.com shiur by Rabbi Shimon Isaacson
Chazal forbade putting fully cooked food on the fire on Shabbat because one could come to stoke the coals. Additionally it is mechzi k’mevashel, it appears as if one is cooking. However, if one satisfies five requirements, then bishul becomes chazara and is permitted l’chatchila.
The five conditions are,
1. the flame must be covered,
2. the food must be fully cooked,
3. it must still be warm,
4. one may not release one’s hold on the pot, and
5. one must have intention to return the pot to the fire.
Two issues arise with reheating food on Shabbat. The food is no longer warm and it is no longer in hand. The Magen Avraham explains that chazara is permitted because it is a continuation of the original act of putting the food on the fire, rather than an initial placement on the flame. The first three conditions create this distinction. Therefore, the Mishna Berura rules that bdi’eved if one does not have one of the last two conditions one may still do chazara.
The Biur Hagra notes a disagreement between Rashi and the Rosh whether the rule of ein bishul achar bishul (cooked foods cannot be recooked) applies only to solid foods or also to liquids. The Rambam holds that it applies equally to both. The Rama takes a middle position and rules that a dry food with liquid gravy may be placed near the fire, but a liquid is prohibited. The Shulchan Aruch rules like Rashi who holds that yesh bishul achar bishul b’davar lach (cooked liquids can be recooked – and therefore, it is asur to do so on Shabat). The Rama notes that the custom is to be lenient and one may return liquid to the fire as long it did not cool down completely. It seems like the requirement of it not cooling down does not belong to the trilogy distinction between chazara and mechzi k’mevashel. The Gra and the Rama maintain that putting cold cooked liquid back on the fire may be a prohibition of bishul d’oraita. Rashi and the Shulchan Aruch hold that this requirement pertains to chazara. According to the Magen Avraham, the condition of lo nitzanzen (not having cooled down) applies equally to liquids and dry foods. The Gra maintains that it only pertains to liquids.
According to the Ran as quoted by the Rama, all five requirements of chazara only apply if one took the food off the fire before Shabbat. Therefore, if the pot was on the flame when Shabbat began and you served from it on Friday night and then put it down, you could still return it to the stove if it did not fully cool down.
The Mishna Berura says there is a basis for this leniency but it is better to be stringent as many poskim disagree. The Shulchan Aruch writes that it is permitted to rewarm dry food on top of a pot filled with food, since it is not a normal way of cooking.
A “kediera” blech is a wide pot filled with water. Some maintain that you can put food on top of this and some say since it does not contain food, it is has the din of a regular blech and has not solved the problem.
The Shulchan Aruch notes that warming food near a fire is permissible because it is not actually putting the pot on the flame. If the food will not reach yad soledet (boiling point) it is permitted.
Can one do chazara by putting food back in an oven on Shabbat? Rav Moshe Feinstein held that to satisfy all five requirements of chazara one would need to use an oven insert to conceal the heating source. Rav Aharon Kotler ruled leniently as long as the knobs are covered.
Does a hotplate have the same din as a stove? Rav Moshe maintained that if one cannot cook on it and it only has one setting one may do chazara. Rav Elyashiv rules stringently against this. One can place an oven rack to make a hefsek kedeirah on a hotplate and then put food on it to rewarm. Differing circumstances and situations may vary. Therefore, all questions should be addressed to a competent Rav for a final decision.