Shabbat, Chapter One, Mishnah Ten
In todays 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 yesterdays mishnah we learned of a case where Bet Shammai agrees with Bet Hillel and in todays mishnah we learn of a case where Bet Hillel agrees with Bet Shammai.
1) Meat, onion[s], and egg[s] may not be roasted unless they can be [fully] roasted while it is still day.
2) 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.
a) Rabbi Elazar says: there must be time for the bottom to form a crust.
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 foods 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.