1) PLACING FOOD INSIDE A "KIRAH" RIGHT BEFORE SHABBOS
QUESTION: The Gemara quotes Rav who explains that when the Mishnah says that one may place food on a Kirah on Shabbos, it means only on top of the Kirah, but not inside the Kirah, for the act of placing food inside the Kirah is more stringent than the act of placing food atop the Kirah.
The Gemara says that if the Mishnah follows the view of Chananyah, it makes sense why one may not place food inside the Kirah (see below). If, however, the Mishnah follows the view of the Rabanan, there should be no difference between placing food on top of the Kirah and placing food inside the Kirah. (This is because one may place food on top of the Kirah only when the coals were cleared away or covered. In such a case, however, where the coals were cleared away or covered, it should also be permitted to put the food inside the Kirah!)
According to Chananyah, what is the difference between placing food inside the Kirah and placing food on top of the Kirah? Rashi (DH Hayinu) explains that since Chananyah permits Shehiyah even when the coals were not cleared away, he cannot permit one to place food inside the Kirah, because the coals are still there and doing so constitutes an act of Hatmanah in a material that adds heat, which is forbidden (see Insights to Shabbos 34:1:a:3).
TOSFOS (DH Ela Iy) asks that placing food inside the Kirah cannot actually be considered Hatmanah in coals, because we find that Chananyah permits placing dough in an oven before Shabbos as long as there is time left before Shabbos for its surface to become crusted (20a). Placing food inside an oven is apparently not considered Hatmanah in a material that adds heat. Why, then, is one forbidden from placing food inside a Kirah according to Chananyah?
ANSWER: Tosfos answers that when the Gemara says that one may not place food inside the Kirah, the Gemara is referring to an act of Chazarah performed on Shabbos, and not to Shehiyah performed before Shabbos. On Shabbos, one may not place food back inside a Kirah because such an act resembles an act of actual cooking (see Insights to Shabbos 36:3:b).
2) HALACHAH: "SHEHIYAH"
OPINIONS: Chananyah and the Rabanan argue whether one may place food that is cooked k'Ma'achal Ben Derusa'i, right before Shabbos, on top of a stove whose coals have not been cleared away or covered. What is the Halachah?
(a) RASHI (DH v'Rav Sheshes) and TOSFOS (DH Amar Rav Sheshes) in the name of RABEINU CHANANEL write that the Halachah follows Chananyah, and one is permitted to do Shehiyah and place food on a stove whose coals have not been cleared away or covered, as long as the food was already cooked k'Ma'achal ben Derusa'i.
(b) The ROSH (3:1) cites many Rishonim (including the RIF and RAMBAM) who rule that it is prohibited to do Shehiyah on an uncovered flame. The Rosh prefers this ruling.
HALACHAH: The SHULCHAN ARUCH (OC 253:1) cites both opinions of the Rishonim. He first cites the opinion of the Rosh, Rif, and Rambam (b). He cites the opinion of Rashi and Tosfos by saying, "And there are those who say...." The BI'UR HALACHAH points out that this wording of the Shulchan Aruch indicates that he rules like the first opinion.
The Bi'ur Halachah adds that we should not reprimand one who is lenient, who relies on the opinion of Rashi and Tosfos, because, as the Rosh writes, "There is a major argument as to how to rule in this matter, and the Jewish people are very attached to the Mitzvah of Oneg Shabbos and will not listen to us if we tell them to be stringent, therefore we should leave them to practice their custom of relying on those who rule like Chananyah." However, for oneself (as opposed to instructing others what to do), one should be stringent and not do Shehiyah on an uncovered flame with a food that is not completely cooked (or with a food that will improve in taste the more it is cooked), unless there are extenuating circumstances that compel him to do so, such as if guests arrived at one's home not long before Shabbos.