One of the thirty-nine Melachot (categories of forbidden activity) that apply on Shabbat is the prohibition of "Tohen," grinding. In light of this prohibition, the question arises as to whether one may cut vegetables into small pieces for a salad on Shabbat. For example, many people prepare "Israeli salad" by cutting fine pieces of cucumbers and tomatoes. Would this be permissible on Shabbat, or does this violate the prohibition of "Tohen"?
The Shulhan Aruch writes, "It is forbidden to cut vegetables very fine." According to the Shulhan Aruch, it seems, there is no basis for allowing the slicing of vegetables into small pieces on Shabbat, and one must make the pieces bigger than he normally does when cutting vegetables on Shabbat.
However, in his Bet Yosef, Maran (author of the Shulhan Aruch) cites the ruling of the Rashba (Rabbi Shelomo Ben Aderet or Barcelona, 1235-1310) that one may slice vegetables into small pieces shortly before the meal in which they will be served. The Rashba draws a comparison between the prohibition of "Tohen" and the prohibition of "Borer" (separating undesirable items from desirable items). He writes that just as Halacha permits separating undesirable food from desirable food in preparation for a meal that will soon be served, similarly, one may slice vegetables in close proximity to the meal. Maran, after citing this ruling, notes that no authorities disputed the Rashba’s position. One might therefore argue that when the Shulhan Aruch codified the prohibition against finely cutting vegetables on Shabbat, he relied on his comments in the Bet Yosef permitting doing so shortly before the meal. He forbade cutting small pieces only when one cuts them for later on Shabbat, but allowed cutting small pieces while preparing for a meal that will soon be served, within a half-hour or so of the meal. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) observed that the prevalent practice in Baghdad followed this lenient reading of the Shulhan Aruch’s ruling.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), however, disagreed with this reading. In his work Or Le’sion (both in vol. 1 and in vol. 2), he contended that since the Shulhan Aruch made no mention of this stipulation, we must assume that he forbade finely slicing vegetables on Shabbat under all circumstances, even just prior to the meal. According to Hacham Ben Sion, one who wants to cut vegetables into small pieces on Shabbat may do so only if he deviates from his normal manner of slicing. For example, if he normally cuts vegetables on a cutting board, he should cut them on Shabbat directly on the counter.
As for the final Halacha, Hacham Ovadia Yosef, in his work Halichot Olam, follows the lenient position, and allows one to cut vegetables into small pieces on Shabbat shortly before the meal at which they will be served. He adds that one who wishes to act stringently to satisfy all opinions "is worthy of blessing," but those who wish to be lenient certainly have a legitimate Halachic basis on which to rely.
Summary: One who wishes to cut vegetables into small pieces on Shabbat may do so if he deviates from the normal manner of cutting (like cutting on the counter, instead of a cutting board), or if he cuts pieces that are larger than the pieces he normally cuts. According to many authorities, one may cut vegetables normally into very small pieces if he does so shortly before the meal at which they will be served, and one may rely on this ruling.
