Whereas a person may not cook food on Shabbat, there is no biblical prohibition against food cooking itself on Shabbat. Therefore a person may place raw meat in a pot on Friday afternoon Erev Shabbat before sunset and allow it to cook through until the following day for the Shabbat lunch meal. Or one may place partially cooked food on the stove on Erev Shabbat before sunset and allow it to cook through so that it will be ready for the Friday night meal.
The only concern the rabbis have with this is that once Shabbat has begun, one may be tempted to accelerate the cooking by turning up the flame. To eliminate this concern, the rabbis required that the coals used in the stoves in Talmudic times be covered over with ashes, katum, to prevent their stoking.
The modern-day equivalent of katum is the aluminum sheet placed over the burners, popularly referred to as the blech. Thus, as long as partially cooked food is put on the blech on Erev Shabbat, it may remain cooking there in preparation for the Friday night meal. Leaving the food in this way on Erev Shabbat is known as shehiyah.
Can food left on the stove on Erev Shabbat as described by the term shehiyah be removed from the blech for serving and returned to the blech on Shabbat, an action called chazarah?
Here, the rabbis are not concerned with turning up the flame, because the blech is already in place. Rather, they are concerned that it should not look as is though one is cooking on Shabbat (mechze kemevashel). They therefore permit chazarah where the following conditions are present: (1) the food must have originally been on the blech on Erev Shabbat; (2) the blech must be in place; (3) the food must be fully (not partially) cooked; (4) it must not have cooled off; (5) from the time it was removed to the time it is returned to the blech it must have been handheld, (6) before removing it, one must have the intention to return it to the blech.
With these conditions present, the rabbis are satisfied that chazarah is no more than an extension of shehiyah performed on Erev Shabbat and as such is permitted. They do not view it as an initiation of cooking on Shabbat, which would be prohibited. It should be noted, however, that chazarah with these conditions present is permitted with solids but not with liquids.
According to the Shulchan Aruch, if one of the above conditions is absent, the food cannot be returned to the blech on Shabbat. All poskim agree that there can be no chazarah without the presence of the first three conditions, but many argue that some or all of the last three are unnecessary.
For example, the Mishnah Berurah mentions that, in a pinch, either condition 5 or condition 6 can be waived. Others, such as Reb Moshe Feinstein, waive, in addition, condition 4 and permit food to be taken out of the refrigerator and put on the blech but away from the flame – so long as the food will not reach a temperature that would burn one’s hand (yad soledet bo), which according to some means 110 degrees Fahrenheit (43 degrees C) and according to others, 160 degrees F (71 degrees C).
Rav Ovadia Yosef concludes that a hot plate does not fall into the category of mechze kemevashel because it is not used during the week. Rav Yosef therefore concludes that food (not liquids) may be taken from the fridge and placed on the hot plate even if it heats up to yad soledet bo.