Gemara

Rav Papa: Dvei Rav Papa bar Aba asked me whether one may knead a dough with milk. I answered 'no.' This was correct.

(Beraisa): We may not knead a dough with milk. If one did, it is forbidden; lest he eat it with meat;

We may not smear an oven (for baking bread) with fat. If one did, the first bread baked inside is forbidden (unless the oven was heated up beforehand).

Pesachim 30a: An oven was smeared with fat. Rabah bar Ahilai forbade all bread that will ever be baked in it, even to eat it with salt, lest it be eaten with Kutach (a common dairy dip).

Question (Beraisa): We may not knead a dough with milk. If this was done, all the bread is forbidden, lest it be eaten with meat. Similarly, we may not smear an oven with fat of the tail (and bake bread). If this was done, all the bread is forbidden, unless the oven was Husak (heated up) before this.

Inference: If the oven was Husak, the bread is permitted!

36a - R. Yehoshua (to his children): Do not knead for me with milk on the first day (of Pesach). Afterwards, you may do so.

Question: The Beraisa forbids this!

Answer #1: Rather, he allowed kneading with honey after the first day.

Answer #2: Really, he allowed kneading with milk after the first day. Ravina permits like an ox's eye. R. Yehoshua permitted like this.

Rishonim

Rif and Rosh (Chulin 38a and 8:21): We may not knead a dough with milk. If this was done, all the bread is forbidden, lest it be eaten with meat. Similarly, we may not smear an oven with fat of the tail. If this was done, all the bread baked until the oven was Husak is forbidden. We conclude that if he did it different than usual, e.g. like an ox' eye. It is permitted.

Rosh: Rashi explains that he made only a small amount, like an ox' eye.

Rambam (Hilchos Ma'achalos Asuros 9:22): We may not knead a dough with milk. If this was done, all the bread is forbidden, lest it be eaten with meat. We may not smear an oven with fat of the tail. If this was done, all the bread baked until the oven was Husak is forbidden, lest it be eaten with meat. If he made the bread in a different form that is recognizable, so he will not eat it with meat or milk, it is permitted.

Hagahos Ashri (Avodah Zarah 5:8): L'Chatchilah, we are concerned for smell. Therefore, if bread was baked with roasting meat, one may not eat it with Kutach. Since it can be eaten without Kutach, this is l'Chatchilah. B'Di'eved, it is permitted. Even though we are concerned for smell, one may bake in our ovens, which are covered above, meat pies with bread. Our ovens are big. However, if fat flowed from the meat pie and was absorbed in the oven, even if the bread is far from the meat pie, one may not eat it even with salt. Therefore, it is good to be careful not to bake any meat pies with bread in an oven, lest the fat drip and he will be unaware. Rather, he bakes the meat pie, fires the oven and (then) bakes bread.

Poskim

Shulchan Aruch (YD 97:1): We may not knead a dough with milk, lest it be eaten with meat. If this was done, all the bread is forbidden, even to eat it by itself. If it was a small amount that is eaten at once, or he changed its form so it is recognizable and he will not eat it with meat, it is permitted. Similarly, we may not bake bread in an oven with fat of the tail. If this was done, it is like bread kneaded with milk.

Beis Yosef (DH Ein): Rabah totally forbade the bread baked in the oven smeared with fat only with salt. It says that he forbade it with salt, for in those days the bread had no salt, and had no taste at all without salt. The Rashba permits if he changed the form of the bread and made small loaves like an ox's eye, for one sees that it is different and will ask, and they will tell him. Sha'arei Dura (60:2) says this and each other Perush (the loaves are shaped like an ox's eye) are true according to the Halachah. He also permits l'Chatchilah baking loaves for Lechem Mishneh for Shabbos with meat pies. Since it is a small amount and it is eaten quickly on Shabbos, it is like an ox' eye.

Taz (1): From here it seems that if a mortar used to pound spices that are used with meat or milk, and once it was used to pound garlic with duck sauce, one may not use the mortar any more, even for spices to be used with meat. This is unlike 96:3, which forbids spices pounded in a meat mortar to be used with milk, but permits with meat. There, the mortar is special for meat, and one will not err.

Pri Chodosh (1, cited in Machatzis ha'Shekel): We are concerned only for Stam bread, which is normally eaten with meat or milk. One does not designate bread for either. Therefore, we decree lest he eat it with meat. Other things, such as a mortar, are normally designated for meat or milk. Even if initially it was (Pareve) just for spices, once he designates it for meat, he will be careful with it. We may not invent new decrees.

Machatzis ha'Shekel: In our land, no one designates a mortar for meat or milk, so the Shulchan Aruch's law applies like the Taz says. If loaves for Lechem Mishneh for Shabbos were kneaded with meat, we permit them with meat. We do not decree lest he eat them with milk, for they are normally eaten with meat on Shabbos.

R. Akiva Eiger (2, citing Minchas Yakov 60:2): The Maharit says that we forbid only if there is concern lest he eat it with meat, but if the only concern is lest he eat it after meat or cheese, we are not so stringent.

Rema: Therefore, our custom is to knead bread with milk on Shavuos, and also with fat for Kavod Shabbos, for this is considered a small amount. Also, it is shaped different than other bread. All the more so, (cheese) blintzes and meat pie are permitted. One may not bake any bread with blintzes or meat pie, lest some of the fat drip onto the bread. If one baked (them together) and the fat dripped under it, it is as if it was kneaded with it. Our custom is to put the meat pie in the opening of the oven. Even if it is a frying pan, we are stringent l'Chatchilah.

Taz (2): This is not if the fat is seen under the bread. Rather, it spread from (under) the meat pie. Hagahos Ashri connotes like this.

Bach (3): The Rema is stringent only when the fat dripped under the bread. He did not understand Hagahos Ashri. Really, in a small oven we forbid even if a pan is underneath (to collect any fat that drips, and without a pan we forbid even in a big oven if there is concern lest fat drip. In our big ovens it is Asur l'Chatchilah, but permitted b'Di'eved.

Shach (2): L'Chatchilah one may not bake bread with blintzes or meat pie, even if the oven is wide and not slanted, lest the fat drip onto the bread. B'Di'eved, we do not assume that it dripped, unless the oven is slanted or the bread is above the meat pie. If we see that fat flowed from the meat pie, even if it did not flow under the bread, the bread is forbidden even b'Di'eved. Hagahos Ashri, Sha'arei Dura and Isur v'Heter all say so. I say that it is because we are unsure whether or not cooking spreads (the contents) throughout the entire Kli, therefore is we do not know whether or not fat flowed, it is a Sefek Sefekah and we are lenient. If the oven is slanted or we see that fat flowed, there is only one Safek. Hagahos Isur v'Heter says in the name of Or Zaru'a that if we did not see fat flow, (even if the oven is slanted) there is a Sefek Sefekah, for even if fat flowed, perhaps he will not eat the bread with milk. The Rema disagrees, for people regularly eat bread with all foods, so this is not considered a Safek. If so, the Rema forbids when fat flows under it, i.e. the meat pie, even if it is far from the bread. He says so in Toras ha'Chatas. The Bach did not look into Toras ha'Chatas, therefore he misunderstood the Rema.

Taz (3): A woman baked cheese blintzes in an oven on its floor, without a Kli, then put a boiling pot of meat in the same place. I permitted. The earthenware of the oven that absorbed from the cheese is no worse than a pot in which milk is cooking that touched a meat pot. Two pots that touched each other are permitted, like the Poskim and Rema (92:8) say. We are stringent about a frying pan only l'Chatchilah.