Gemara

96b (Mishnah): If a thigh was cooked with the Gid ha'Nasheh inside, if it is Nosen Ta'am (imparts taste) to the thigh, the thigh is forbidden.

(Shmuel): This is only if the thigh was cooked (with water), If it was roasted, it is permitted (but one may not eat the meat around the Gid).

Question: Rav Huna taught that if a kid that was roasted with its Chelev, even the tip of the ear is forbidden! (The Chelev gives Ta'am to the entire kid.)

Answer: Chelev is different. It bubbles up and spreads.

Question: Does Chelev really forbid the rest of the animal?!

A case occurred in which a kid was roasted with its Chelev, and R. Yochanan permitted it, except for the areas around the Chelev!

Answer: It was a thin kid.

108a (Mishnah): If a drop of milk fell on a piece of meat in a pot, if the milk gives Ta'am to the piece, it is forbidden;

If the pot was stirred, if the milk gives Ta'am to the contents in the pot, it is all forbidden.

Pesachim 75b (Mishnah): If juice dripped onto the oven (it is cooked by it) and splattered back onto the Korban Pesach (which must be only roasted), one must remove Kedei Netilah (more than just a later) from that place (for the drop gets absorbed);

Rishonim

Rambam (Hilchos Ma'achalos Asuros 15:32): If a kid was roasted with its Chelev, one may not eat even from the end of the ear, for the Chelev is absorbed in the limbs and improves them and gives taste. Therefore, if it was lean, and there was only a little Chelev of the kidneys and of the stomach, like one part in 60, he peels and eats, until he gets to the Chelev.

Rosh (Chulin 7:24): One does not literally eat until reaching the Gid, for he must leave Kedei Kelipah (a layer that can be peeled), and even Kedai Netilah, i.e. a finger-width, like we say in Pesachim. One could distinguish and say that gravy penetrates more than the Gid. However, we never find that Kelipah suffices for something roasted. Kelipah is never mentioned regarding roasting. Rav Huna says that the Chelev forbids the entire animal, for it totally penetrates.

Rosh (ibid.): We find that when cold falls into hot (and one of them is Isur), the opinion that the bottom overpowers totally forbids. It is not enough to remove a Kelipah. When we salt much meat together on wood, and the brine (with blood) flows down, sometimes we find a piece partially in the brine. We cut off the part in the brine (for it absorbed blood), and permit the rest. We do not say that it penetrates upwards, the way we say here. This is because blood does not penetrate. Even when it need not go upward, we say that it is smooth and it is not absorbed. All the more so, it does not penetrate upwards. Even if the part in the brine has fat, we do not say that it becomes forbidden like a Neveilah and penetrates upward and forbids, according to R. Efrayim, who does not say that a piece becomes forbidden like a Neveilah. Even without this, a piece that absorbed Isur forbids only where it itself could have spread to there and given taste. When a drop of Chelev falls on a piece, its taste spreads to all the pieces. Even though the drop could not give taste to all the pieces, the piece it fell onto helps it. Blood cannot penetrate upwards, therefore fat forbidden due to the blood does not penetrate upwards to forbid.

Rosh (ibid.): R. Tam says that the lean kid (roasted with its Chelev) was permitted because Chelev of a lean animal does not penetrate. Alternatively, it had a little Chelev, and it was Batel in 60.

Beis Yosef (YD 105 Sof DH u'Mah): The Tur wrote the law of a kid roasted with its Chelev. He did not specify whether we permit when it is lean because the Chelev does not penetrate, or because there is 60 times as much Heter.

Gra (YD 105:31): The first answer agrees with the latter. On 108a, we say that if the drop does not give taste, it is Batel and everything is permitted.

Ran (Chulin 34b DH Gedi): The Ro'oh disagrees, and says that Chelev that penetrates is never Batel through roasting. It is Batel only through cooking, for the liquid evenly distributes the Isur. In roasting, the Isur penetrates by itself. Perhaps a lot gathered in one place. Therefore, it is not Batel even in 60. According to this, we should not rely on 60 also regarding salting! The Gemara connotes unlike this.

Beis Yosef (YD 105 DH u'Mah she'Chasav Rabeinu): It seems that the Ran disagrees with the Ro'oh. Also the great Poskim disagree, i.e. the Rambam, Rashba, Tosfos and Rosh.

Poskim

Shulchan Aruch (YD 105:5): Roasted meat forbids only Kedei Netilah in cases like the thigh and its Gid, i.e. something lean without strength to penetrate into the entire piece. However, if a fatty kid was roasted with its Chelev, if the entire kid is not 60 times all the Chelev, one may not eat even from the tip of the ear. Since it is fatty, it totally penetrates. If it is lean, even if there is not 60 times all the Chelev, it forbids only Kedei Netilah. The Chelev of a lean animal is weak; it does not penetrate.

Beis Yosef (DH veha'Rashba): The Rashba and Ramban hold like R. Tam, that Chelev of a lean animal does not penetrate. Therefore, the kid is permitted even if it is not 60 times the Chelev.

Gra (32): The primary answer (why a lean kid is permitted) is because lean Chelev does not penetrate. The answer that there is 60 times as much Heter is difficult, for the Gemara connotes that Shmuel agrees to this, and he holds that Min b'Mino forbids any amount (Avodah Zarah 73b)! On 97b, thighs were salted with the Gid ha'Nasheh. There was not 60, for Ravina forbade them. Even so, we conclude that they are permitted, because salting is only like roasting!

Shulchan Aruch (ibid.): Even if a lean piece of Isur was roasted with a fatty piece of Heter, the Isur totally penetrates.

Taz (10): The Beis Yosef says that the Tur learned this from smells. The one who forbids due to smells, he forbids even if the Heter is fatty and the Isur is lean. The Heter fattens the Isur, and it gives smell to the Heter. This is difficult. The Terumas ha'Deshen distinguished between this and the law that what absorbed Isur can forbid only where the Isur itself could have reached. Smell is different, because fat carries the smell. This shows that the stringency is due to the smell. Since we do not forbid b'Di'eved due to smell, why should we forbid here b'Di'eved? Only smell transfers (to the entire piece of Heter), and it does not forbid b'Di'eved! A forbidden absorption can forbid only where the Isur itself could have reached, even when the pieces touch! Surely the Tur does not forbid here due to smell, for he said the same regarding salting. Smell does not apply there! Darchei Moshe said that here we discuss lean Chelev in the Isur. It is called lean because it does not penetrate. Only in this case we say that the fat penetrates, for it is called Chelev. This is difficult, for we learn from smell. There we discuss lean meat, but not lean Chelev! Also, we did not mention Chelev here, only Isur! Some answer that here, the Isur is forbidden due to itself. Even if it is lean, it emits juice of Isur, and Heter fat mixes with it. There, when Heter absorbed Isur, the fat of the Heter does not take the Isur with it. This is wrong. Without liquid, Isur forbids Kedei Netilah of what it touches. Its strength fades after this. An absorbed Isur can penetrate through the entire piece. It cannot go further without liquid. Even though there is fat where the Isur is, the fat does not help the Isur pass its limit. All the more so, Isur itself, which forbids until Kedei Kelipah, Heter fat does not help it to go further!

Defense (Nekudas ha'Kesef): The answer they gave is correct. Since the Isur is forbidden due to itself, the Heter fattens it, and (it is now a fatty Isur, so) it fattens (and forbids) the Heter.

Gra (33): Even Levi permits only because he holds that smell does not matter.

Taz (10): We also learn from a piece of meat partially in brine. Fat in the meat does not help the brine penetrate, since the brine itself cannot penetrate. The Beis Yosef said that this is because blood is smooth. This is astounding. In both cases, fat does not change the nature of blood.

Rebuttal (Nekudas ha'Kesef): The Tur agrees that salting is different. Fattening does not apply to it. The Tur discusses only roasting. The Maharshal, Prishah and Bach resolve the Tur like this.

Taz (10): It seems that the text of the Tur should say 'even if the Isur is fatty and the Heter is lean.' I.e. not only in one piece, Chelev penetrates throughout the entire piece. Rather, even when there are two pieces, a fatty Isur and a lean Heter, the Isur penetrates through the entire Heter. The Maharshal (Chulin 7:45) says that a piece with its own blood is considered like a piece forbidden due to absorptions.