The question of the timing of the cutting of a loaf of bread, roll, etc., is the subject of a machloket in the gemara[1] with regard to the preference throughout the week of reciting a beracha on a whole loaf of bread. According to Rabbi Chiya, betzi’at hapat (the breaking of the bread), which should be performed in accordance with a number of halachot,[2] is to be done as one is reciting the beracha. Rava argues that what is important is that the bread should still be whole at the conclusion of the beracha, and one should therefore not cut off a piece until after the beracha is complete. The gemara concludes that we accept Rava’s opinion.[3]
The gemara’s discussion, as we noted, is referring to cutting bread on a weekday, when the stakes are low, as there is no requirement to use a whole loaf. On Shabbat, when one is required to have lechem mishneh,[4] it is even more important that the challot remain intact throughout the beracha.
The Rosh[5] writes that although the separating off of the part of the bread to be eaten is to be done after the beracha, one should make a significant but only partial cut of the loaf before the beracha. The reason is to minimize the delay between the end of the beracha and the eating of the bread.[6] The Rama[7] makes a practical distinction based on the heightened level of importance of wholeness on Shabbat, ruling that partial cutting to expedite matters is justified during the week, when the wholeness of the loaf is only a preference, but on Shabbat, when wholeness is crucial, one should not cut it at all. (If one did cut it, but left it intact enough that if one were to lift the loaf by the smaller part, the weight of the larger part would not cause it to be severed, the loaf is still considered fundamentally whole as far as making a beracha is concerned.[8])
Poskim[9] recommend a compromise for breaking bread on Shabbat that most people follow to at least some degree. One should scratch a line on the challa at the place where he plans to cut, thereby saving time when he does later cut the challa. Many people do more than scratch the challa and actually make a small cut. (This seems to make more practical sense than merely making a scratch, as having to position the knife exactly at the place of the scratch takes more time than to start a new cutting.) On the other hand, we do not make a significant cut, due to the concern the Rama addressed that too large a cut could make the challa be considered two challas that are only nominally connected, especially if the cut deepens while handling the bread. In any case, any minhag along these lines is acceptable.
Due to the above, using matza for the second “loaf” of lechem mishneh can sometimes cause challenges. (We will just mention in this context the fact that using matza is a problem for Sephardim – and thus a problem when one has Sephardi guests – as Sephardic poskim do not consider matza to be bread, and Sephardim actually recite Mezonot on it.)[10] One should hold both loaves during the beracha,[11] and in the daytime, the loaf that one is cutting should be on the top.[12] It requires some care to hold a nice-sized challa on top of a matza without breaking the matza. Preferably, no part of the lechem mishneh should break,[13] although we generally assume that if the piece that broke off is less than 2% of the “loaf,” it is not a problem b’di’eved.[14] Regarding a situation in which the matza cracks but is not severed, we would again apply the test of connectedness mentioned above.
Despite these challenges, those Ashkenazim who want to use matza may do so, but they would be wise to be careful in handling it. Even at seuda shlishit, one should be careful to keep the loaves intact until after the beracha, as the poskim say that one should have lechem mishneh for that meal as well.[15] However, there are opinions cited in the Shulchan Aruch[16] that bread is not required at all at seuda shlishit. Even if bread is required, the Rama[17] rules that it is acceptable, although not preferable, to have only one whole loaf at seuda shlishit.