The basic law of "Borer" – separating different items from one another on Shabbat – allows one to separate as long as: 1) he removes the Ochel (edible food) from the Pesolet (undesirable food), and not the other way around; 2) this is done by hand, and not with a utensil designated for separating; 3) this is done for immediate use.
In light of these guidelines, the question arises as to the permissibility of peeling an onion on Shabbat. Even assuming that one peels the onion by hand and for immediate use – such as in preparation for a dish that will be served at a meal about to take place – peeling involves removing the Pesolet – the peel – from the Ochel – the desirable part of the onion. Seemingly, then, peeling an onion fails to satisfy one of the requirements of separating on Shabbat, namely, that one remove the desirable substance from the undesirable substance. This question applies not only to peeling onions, but also to peeling other foods, such as garlic and bananas. In all these cases, one removes an undesirable peel from a desirable food, and the question thus arises as to whether this may be done on Shabbat, even if one peels by hand and in preparation for a meal that will soon be served.
This question is addressed by the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), at the end of Siman 321, where he writes that one may peel an onion on Shabbat, since removing the peel is the only way to access the edible part of the onion, and this is thus considered the normal method of eating. It is noteworthy, however, that he accepts the premise that this indeed constitutes "Borer." One may have argued that since the peel and the edible part of the onion are not actually mixed together, removing one to access the other would not qualify as "separating." The Rama does accept this viewpoint, and acknowledges that peeling an onion indeed qualifies as Borer, and is thus permissible only if it is done by hand and for immediate use. Hence, one may peel an onion while preparing a salad for a meal about to be served, but one may not peel an onion on Shabbat morning in preparation for Se’uda Shelishit. Likewise, one may not peel a banana on Shabbat morning to make a fruit salad for Se’uda Shelishit.
Rabbi Moshe Halevi (Israel, 1961-2001), in his Menuhat Ahaba and in his Tefila Le’Moshe, applies this ruling to a pile of clothing. For example, it often happens that children’s coats are piled on top of one another on a hook, and one might need to remove several jackets in order to access his or her jacket. Rabbi Moshe Halevi observes that this situation resembles that of the onion, where one must remove the outer layers in order to access the desirable inner layer. Halacha follows the view that "Borer" applies even to utensils and clothing, and thus it would apply to separating jackets, as well. Therefore, just as in the case of the onion, it would be permissible to remove jackets in order to access one’s jacket, but only if this is needed for immediate use. If, however, a mother wants to prepare her children’s coat or other article of clothing at night for the next morning, it would be forbidden to remove garments from a pile to access the garment she wants to prepare for the next day. This kind of "separating" is permissible only if it is done for immediate use, but not if it is done in preparation for the next morning or later in the day.
Summary: It is permissible to peel an onion, garlic or banana to prepare it for immediate use, such as for a meal that will soon be served, but not to be served later. Similarly, if one’s jacket is underneath a pile of other jackets, one may remove the jackets to access one’s own jacket only if he needs it for immediate use.
