Commentary for Shabbat 1:10
אֵין צוֹלִין בָּשָׂר, בָּצָל, וּבֵיצָה, אֶלָּא כְּדֵי שֶׁיִּצּוֹלוּ מִבְּעוֹד יוֹם. אֵין נוֹתְנִין פַּת לַתַּנּוּר עִם חֲשֵׁכָה, וְלֹא חֲרָרָה עַל גַּבֵּי גֶחָלִים, אֶלָּא כְּדֵי שֶׁיִּקְרְמוּ פָנֶיהָ מִבְּעוֹד יוֹם. רַבִּי אֱלִיעֶזֶר אוֹמֵר, כְּדֵי שֶׁיִּקְרֹם הַתַּחְתּוֹן שֶׁלָּהּ:
Meat, onion, and egg are not to be roasted (on Sabbath eve) unless they can be roasted while it is still day [as the food of Ben D'rosa, i.e., one-third cooked, at which point it is edible, and there is no reason to decree "lest he stir the coals"]. A loaf is not to be placed in the oven before dark nor a chararah [a kind of cake] on coals, unless its face [i.e., its upper surface] forms a crust [(this being the beginning of baking)] while it is still day. R. Eliezer says: Until its lower surface [on the clay of the stove] forms a crust. [For it bakes first, before the upper surface, and this is sufficient. The halachah is not in accordance with R. Eliezer.]
Bartenura on Mishnah Shabbat
English Explanation of Mishnah Shabbat
Bread may not be put into an oven just before nightfall, nor a cake upon coals, unless its surface can form a crust while it is still day. Rabbi Elazar says: there must be time for the bottom to form a crust.
In today’s mishnah we learn that Bet Hillel agrees with Bet Shammai that food may not be put into an oven on Friday in order for it to cook on Shabbat, unless it has already been fully cooked before Shabbat begins.
Note that in yesterday’s mishnah we learned of a case where Bet Shammai agrees with Bet Hillel and in today’s mishnah we learn of a case where Bet Hillel agrees with Bet Shammai.
We can explain both of these sections together, since they are both in essence the same halakhah. Bet Hillel does not allow food to be left in the oven if the food is uncooked lest someone rakes the coals in the oven in order to increase the heat and cook the food faster. Since unlike the items in the previous mishnayot, the person will want her food on the Shabbat itself there is a fear that she will do something to hurry the food’s cooking. Raking the coals is forbidden on Shabbat under the category of lighting a fire. Hence, to “keep one away from sin” it is prohibited to even have uncooked food in the fire.
Cooking cakes or bread involved placing them on the sides of ovens. The bottom would crust before the top. Rabbi Elazar is therefore more lenient. He allows leaving the bread in the oven as long as the bottom has already crusted over, which will be before the top.