There is a litvishe/yeshivishe minhag to eat liver on the Shabbat morning seudah as first plate, instead of fish. Mishna Berura 242:2 says in the name of Magen Avraham / Tikunei Shabat that it is good to eat fish all three times.
What is the source of this minhag? How can it be halachically strong enough against the Mishna Berurah, which is usually followed by them (in the lack of contradicting sources)?
Related:
It is possible that this minhag is based upon the practice of Rabbi Abba as found in Shabbat 119a which says:
Rabbi Abba bought thirteen plain staters [astirei peshitei] worth half a zuz of meat from thirteen butchers in deference to Shabbat, so that he would have various types of fine meat. And he would place the meats at the door hinge at the entrance to his house to hurry to bring another type of meat. And he said to the cooks, in order to rush them: Hurry and prepare it, hurry and prepare it.
This is mentioned in Otzar Dinim v'Minhagim in connection with foods that are good to eat on Shabbat. The meat described as בשר מן המובחר may be referring to liver.
It is sort of a play on words of:
אמרו עליו על שמאי הזקן כל ימיו היה אוכל לכוד שבת
That liver is also כבד.
