Gemara

41a (Beraisa): If one wants to eat a radish and an olive, he blesses on the radish, and exempts the olive.

Answer: The case is, the radish is primary. (He eats the olive just to weaken the sharpness of the radish.)

44a (Mishnah): If one was served first salted food, and bread with it, he blesses on the salted food and exempts the bread, for it is Tefel (secondary) to it.

The general rule is, if one eats an Ikar (primary food) and a Tefel food together, he blesses only on the Ikar, this exempts the Tefel.

Question: Do we ever find that salted food is Ikar and bread is Tefel?!

Answer (Rav Acha brei d'Rav Avira): The Mishnah discusses Peros Ginosar.

Rishonim

Rambam (Hilchos Berachos 3:7): What is the case of a Tefel that was not mixed? If one needed to eat salted fish, and he ate bread with it lest it harm his throat and tongue. He blesses on the salted food and exempts the bread, for the bread is Tefel to it. The same applies to all similar cases.

R. Yonah (32a DH b'OChlei): Peros Ginosar are very sweet. One eats them only with much salt, lest the sweetness harm him. One eats them with bread, so the bread is Tefel to the fruits. Therefore, he blesses on the Peros and exempts the bread.

Tosfos (44a DH b'Ochlei): The Mishnah discusses one who eats Peros Ginosar, and afterwards salted food, to revive him from weakness due to their sweetness. The salted food is Ikar, and the bread afterwards is Tefel, since it is only due to the salted food. Why don't we say that the fruits are Ikar, and the salted food is Tefel? He should bless on the fruits, and exempt everything else! We can say that he did not eat the fruits in that Ma'amad (session). Alternatively, when he ate the Peros he did not have salted food and bread. He did not know that he will become weak due to the sweetness.

Poskim

Shulchan Aruch (OC 212:1): Even bread, which is more important than everything else, if it is Tefel, e.g. he ate salted fish and eats bread with it so it will not harm him in his throat, he blesses on the fish and exempts the bread, since it is Tefel.

Taz (2): The Maharshal challenged the Tur, for the Gemara connotes that bread is Batel to the salted food only if he ate the salted food after Peros Ginosar, but if he eats bread with salted fish, it (the bread) is Ikar. Why did the Tur say that the bread is Tefel? I say that the Gemara discusses one who has no desire for the salted food. He eats it only due to the Peros. The Tur and Shulchan Aruch discuss one who wants to eat the salted fish, just he fears lest it harm his throat, so he eats a little bread with it. Surely, if one eats herring with bread, the bread is primary. It is the Ikar of his meal. The herring accompanies it. The Shulchan Aruch says 'lest it harm his throat' to show that he would prefer to eat only the fish, if it were possible.

Bach (2): There is a bigger Chidush regarding bread. When he ate a radish and an olive, he began eating only for the radish. The olive is only to blunt the sharpness, so it is Tefel. Rather, even bread, which is most important, if it was Tefel (one does not bless on it). The Rambam and Semag write like this. The Gemara asked how bread can be Tefel, and answered that he ate Peros Ginosar. This connotes that if not, rather, he ate salted food and bread with it, the bread is Ikar and one blesses on it. The custom is to accompany bread with herring, and bless on the bread. This is difficult for the Tur, who says that he blesses on the fish and exempts the bread, which is Tefel. Also the Maharshal asked this. This is not difficult for the Rambam and Semag. They did not say 'one who eats salted fish', rather, 'if he needed to eat salted fish.' I.e. we do not discuss one who needs to eat bread, and this is his primary meal; the salted fish accompanies the bread. Then, he blesses on the bread. Rather, we discuss one who does not need to eat bread at all. Rather, he needs to eat salted fish for some reason. The Rambam holds that the Gemara answered that we discuss Peros Ginosar, or any similar case in which he does not need to eat bread. Rather, he needs to eat salted food for some reason, e.g. he ate or drank something that would weaken him if he will not eat salted food. He eats bread so the salted food will not harm his throat. Then, surely the salted food is Ikar.

Bach (DH Aval): The Tur connotes that we discuss one who eats bread and salted fish. This is his primary meal. Even so, he blesses on the fish and exempts the bread. This is wrong. Perhaps the Tur relies on what he wrote 'he eats bread with it so it will not harm his throat' to teach that he does not need to eat bread, rather, he needs to eat salted fish, like the Gemara concluded. He did not make this as clear as the Rambam and Semag did.

Bach (DH Aval): Tosfos connotes that he blesses on the salted food sheha'Kol, and exempts the bread. All the Poskim say so. R. Yonah says that he blesses on the Peros and exempts the bread. I.e. we conclude that he blesses on the food that caused him to eat the salted food. He blesses on the Peros, which are Ikar, and exempts the bread, which is Tefel. One does not bless on the salted food at all to exempt the bread. This is like the Gemara initially asked 'can salted food be primary, and bread Tefel?!' The answerer agreed. The case is, the Peros are more important than the bread. This is like Rashi explained. Based on this, if after eating the Peros they brought salted food and bread, one blesses on the bread, and not on the salted food.

Bach (DH v'Achti): Above (Siman 202), the Tur connotes that if one's throat hurts and he eats oil for Refu'ah through bread, even though he has bread only so the oil will not harm him, he blesses on the bread because it is considered Ikar. Regarding salted fish, we consider the bread Tefel! This is not difficult for Rashi and R. Yonah. They hold that nothing is more important than bread except for Peros Ginosar. This is difficult for Tosfos and the Poskim. This requires investigation.

Magen Avraham (2): Normally, one eats bread only if he is hungry, and if he is, bread is primary! Therefore, the Shulchan Aruch answers that he is not hungry. He eats it only due to the salted food. They (Tosfos) connote that if he knew that he will become weak, even if the salted food was not in front of him, the Berachah on the Peros exempts it. The Shlah says that if one eats bread to sweeten the drinking, he blesses on the drink and exempts the bread if he explicitly intended for this, or normally does so, for then it is as if he explicitly intended. If not, he must bless (on the bread). The Ba'al ha'Ma'or asked 'what is the Chidush that bread exempts matters due to the meal? We always bless on the Ikar and exempt the Tefel!' He answered that even if he did not intend for the Tefel when he bless on the bread, it is exempt. We must say that this is only for bread, since a meal is fixed on it and one intends for everything they will bring to him. Regarding other cases of Ikar and Tefel, one must intend for the Tefel. If he did not intend (for the bread), he blesses sheha'Kol on the bread.

Machatzis ha'Shekel (DH v'Im): The last words of the Magen Avraham refer to what he said above, according to Tosfos. If he did not expect to become weak, and the salted food was not in front of him, he blesses sheha'Kol on the bread if he eats it before the salted food.

Mishnah Berurah (3): If one also wants to eat bread, even if he eats it with fish such as herring, even if he prefers the fish, he blesses on the bread and exempts the fish.

Kaf ha'Chayim (3): Hilchos Ketanos (2:59) says that if one ate bread to satiation, he blesses Birkas ha'Mazon.

Magen Avraham (3): The Shlah says that when one eats bread to sweeten the drinking, he washes his hands without a Berachah. Teshuvas ha'Rema says that he need not wash.

Kaf ha'Chayim (9): He washes without a Berachah.

Mishnah Berurah (177:3): If one does not want to eat bread, and he eats it only to exempt other foods from Berachos, it is not clear whether or not it exempts them. They are not Tefel to the bread, since he does not want to eat it, and all the more so is he eats less than k'Zayis of bread! Or, perhaps since people normally fix a meal on bread, it always exempts them. Therefore, it is best not to eat bread at all in such a case. However, on Shabbos and Yom Tov it is a Mitzvah to eat a k'Zayis of bread, therefore bread is important and other foods are Tefel to it.