As we have learned, if food is not adequately cooked (whichever definition of “adequately,” as defined in the previous section, is adopted), one may not leave it on the fire before Shabbat begins, because one may come to turn up the flame on Shabbat. However, one may do so if the flame is covered. Covering the flame, thus reducing its heat, serves as a reminder not to raise the flame later. Furthermore, were one to forget and go to turn up the flame, he would see that the fire is covered and remember that this is forbidden, and thus refrain from proceeding. Therefore one may leave foods that are not adequately cooked on an electric plata, since its heating elements are covered.
One may also cover the fire by placing a blekh (metal sheet) over the burners and leaving adequately cooked foods on it, since covering the fire is a clear indication and reminder that it is Shabbat, effectively preventing one from forgetting and turning up the flame. Ideally, it is also recommended to cover the knobs that control the flames of the burners.
Although one may put foods on a blekh or plata even if they are not fully cooked, le-khatĥila it is better that all the foods be fully cooked before Shabbat begins. As long as they are not fully cooked, any action that will make them cook faster is prohibited by Torah law. For example, if one removed the cover on a pot containing food that is not fully cooked, one may not replace the cover. Similarly, one may not move the pot to a hotter place on the plata (as explained in section 3). Therefore, it is preferable to leave only fully cooked foods on a plata (SSK 1:72).
. In the times of the Sages, foods were cooked and heated in a stove that contained coals. The Sages decreed that if the coals were removed or covered with ash, it was permitted to leave food there even if it was not yet edible. Nowadays, since we do not cook on coals but rather on gas or electric burners, there is nothing truly similar to removing the coals. Nevertheless, the process of covering the fire with ash can be replicated simply by covering the flame. The purpose of covering the coals was to reduce their heat without putting them out entirely, so that one could still cook with them. The Sages were not worried that one would forget it was Shabbat and feed the fire, because the fact that the coals were covered would remind him that it was Shabbat and this is forbidden. Covering a gas flame with a blekh serves the same purpose; it reminds people that it is Shabbat, so there is no concern that they will raise the flame. Therefore, it is not necessary to cover the knobs on the stove. Indeed, SSK ch. 1 n. 218 (based on MB 253:14) states this, as do Or Le-Tziyon 2:17:2; Shvut Yitzĥak vol. 2, p. 21 in the name of R. Yosef Shalom Elyashiv; and R. Shlomo Zalman Auerbach as cited in Me’or Ha-Shabbat vol. 2, p. 628. This is also recorded by Igrot Moshe OĤ 1:93, which states the key is to cover the flame, even though le-khatĥila one should cover the knobs as well. Shevet Ha-Levi 3:49 maintains that the primary requirement is to cover the knobs in order to prevent the possibility of raising the flame, but requires covering the fire as well. Menuĥat Ahava 1:3:1 states that it is sufficient either to cover the fire or to cover the knobs. See n. 13 above, which mentions an even more lenient position, but we do not follow this.