סימן פ סעיף כא

אָסוּר לַחְתּוֹךְ בְּצָלִים וּשְׁאָר יְרָקוֹת אֶלָּא דַּוְקָא סָמוּךְ לַסְּעוּדָּה וְגַם אָז לֹא יַחְתְּכֵם דַּק דַּק מְאֹד.

It is forbidden to cut up⁹¹The author indicates that unless it is needed for the meal, it is forbidden to cut them even in larger slices. Later poskim maintain that cutting vegetables is not considered “grinding” unless it is done very fine. (Igros Moshe) onions and other vegetables, except immediately before a meal and even then, they should not⁹²However, the Rema 321: 12, Magein Avraham, Peri Megadim, Vilna Gaon, Shulchan Aruch Harav, Mishnah Berurah and Aruch Hashulchan maintain that it is permitted to cut them very fine before the meal. Rav Moshe Feinstein ruled that even a scrupulous person need not be machmir in this regard. However, this is valid only when using a regular knife, but regarding the use of a chopping knife (specially designed for chopping), Biur Halachah is not sure that it is permitted even before the meal, for it may be similar to selection (boreir) with a strainer which is not permitted, even near the meal. Aruch Hashulchan 321: 9 permits using a chopping knife, but Rav Moshe Feinstein ruled that it is befitting to be machmir if the necessity is not great. be cut very fine.⁹³Cooked meat may be cut with a knife into very fine pieces. (Shulchan Aruch 321: 9)

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)