Shabbos, Chapter 1, Mishnah 10. This mishnah deals with actions that may not be performed on Erev Shabbos close to nightfall, even according to Beis Hillel.
Roasting meat, an onion and an egg:
"Ein tzolin basar batzal u'veitzah, ela kedei sheyitzalu mib'od yom" - a person may not roast meat, an onion or an egg on Erev Shabbos unless they will be roasted before Shabbos begins. The basis of the prohibition is the concern that he may stir the coals - that a person will come to stir the coals on Shabbos in order to make sure that his food will be ready in time.
But once they are roasted while it is still day, in the language of the mishnah, one may leave them on the fire even as Shabbos comes in.
What is the measure of "sheyitzalu mib'od yom"?
The Gemara sets the measure at the level known as 'ma'achal ben D'rosai'. Ben D'rosai was a well known bandit who was accustomed to eating food that was not fully cooked, even when it was half raw and barely fit to eat, and that was enough for him. Once the food has reached this level, the concern that a person will stir the coals is reduced. The Rishonim disagree about the measure of this level:
The view of Rashi: one third of the normal cooking.
The view of the Rambam: one half of the normal cooking.
Placing bread in the oven and a cake on the coals:
The mishnah continues: "Ein nosnin pas la'tanur im chasheichah, v'lo chararah al gabei gechalim, ela kedei sheyikrom paneha mib'od yom" - one may not place bread in the oven at the time close to nightfall, and likewise one may not place a chararah (a kind of cake that they used to bake directly on the coals) on the coals, unless its surface will have time to form a crust before Shabbos begins. Once it has reached this stage of baking, it is permitted.
The view of Rabbi Eliezer:
Rabbi Eliezer says: "Kedei sheyikrom hatachton shelo" - in his opinion the decisive factor is not the crusting of the surface of the bread, but the crusting of the underside of the bread or the chararah.
The commentators disagree in understanding his view: who is being lenient and who is being stringent, and essentially, which one forms first:
If the underside crusts first, it turns out that Rabbi Eliezer is being lenient.
And if the underside crusts later, it turns out that Rabbi Eliezer is being stringent.
In summary: in this mishnah we learned the actions that are forbidden on Erev Shabbos close to nightfall out of concern that one may stir the coals: roasting meat, an onion and an egg - until they are roasted to the measure of 'ma'achal ben D'rosai' (one third cooked according to Rashi, one half according to the Rambam); and placing bread in the oven and a chararah on the coals - until its surface forms a crust while it is still day, and according to Rabbi Eliezer until the underside forms a crust, and regarding this the commentators disagree whether his words are a leniency or a stringency.