In the previous Halachot we have discussed that one of the thirty-nine forbidden works on Shabbat is grinding. It is thus forbidden to grind spices on Shabbat. There is, however, a permissible method to grind black pepper and the like on Shabbat which is by implementing two variations, i.e. by grinding using the handle of a knife (and not the side that cuts) and by doing so in a regular bowl (and not within a mortar).
Just as it is forbidden to grind spices on Shabbat, it is likewise forbidden to slice vegetables very finely on Shabbat, as the Gemara (Shabbat 74b) states, “Rav Papa said: One who slices spinach very finely on Shabbat is liable for transgressing the prohibition of grinding.” This is because slicing vegetables finely is similar to grinding and it is therefore forbidden.
The Permissible Method to Slice Vegetables
There is, nevertheless, a distinction between slicing vegetables and actual grinding on Shabbat, as we shall now discuss:
The Rashba writes in one of his responses (quoted by the Bet Yosef in Chapter 321) that when the Gemara states that it is forbidden to slice spinach, this only applies when one has in mind to eat the spinach at a later time, such as one hour later. However, this is permissible when one is slicing the vegetables in order to eat them immediately, for the Torah does not dictate whether one must eat one’s food in large or small pieces.
This means that the only slicing of vegetables similar to grinding which the Torah forbids is only when one slices a vegetable to be eaten at a later time. However, if one wishes to eat the vegetable immediately and slices it in order to do so, the prohibition of grinding does not apply whatsoever.
Maran Ha’Bet Yosef derives from here that one may prepare a “combination of various vegetables,” i.e. what we call a vegetable salad, even by slicing the vegetables finely, as long as one has in mind to eat the salad immediately.
Nevertheless, the Bet Yosef concludes that as an added stringency, it is preferable for one to tell his family to slice the vegetables in a somewhat large fashion and not extremely finely and to then eat the salad immediately in order to stay far away from transgressing a prohibition.
Therefore, one should only prepare a salad close to the beginning of the Shabbat meal just as we have written in the laws of selecting on Shabbat that one may only select or slice vegetables for a salad in close proximity to the meal, i.e. within a half-hour of the beginning of the meal. One must be careful not to begin selecting or slicing vegetables for a salad long before the meal, for this constitutes a Torah prohibition of grinding (or selecting), for this is only permissible when this is being done for immediate use which is within a half-hour of the beginning of the meal. Additionally, there is only room for leniency when there is a necessity to slice a large amount of vegetables in which case one may act leniently within a half-hour of the meal; however, if only a small amount of vegetables are required, one should not slice them until several minutes before the meal.
The Magen Avraham (Chapter 321, Subsection 15) and many other Poskim write that one may only begin slicing vegetables for a salad or selecting foods for the Shabbat meal only when the men have left the synagogue and are already on their way home, for only this can be considered “immediately” prior to the meal but not before this point.
Summary: One may not slice vegetables for a salad on Shabbat unless this is being done for immediate use which is a maximum of a half-hour before the beginning of the meal. When this is the case, one may even slice the vegetables very finely using a knife and a cutting board (see Chazon Ovadia-Shabbat, Volume 4, page 265).