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)?
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