If one forgot to add the paragraph of "Reseh" before "U’bneh Yerushalayim" in Birkat Ha’mazon on Shabbat, and he realizes his mistake immediately upon concluding the Beracha of "Boneh Yerushalayim," he can recite right there and then a special Beracha, "Baruch Ata…Asher Natan Shabbatot Li’mnuha…" This Beracha appears in some Siddurim, and may be recited in this case in lieu of "Reseh." In the parallel case on Yom Tob, where one forgot to add "Ya’aleh Ve’yabo" and realizes his mistake right after "Boneh Yerushalyaim," he recites the Beracha, "Baruch Ata…Asher Natan Yamim Tobim Le’Yisrael…"
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Hukat, ruled that the Beracha of "Asher Natan" may be recited in lieu of "Reseh" or "Ya’aleh Veyabo" only if one had not yet recited the word "Baruch" of the next Beracha. Once the person began the next Beracha – even if he just recited the word "Baruch" – he can no longer recite "Asher Natan," and must therefore return to the beginning of Birkat Ha’mazon. This is also the position taken by Rabbi Moshe Levi (Israel, 1961-2000), in his work Birkat Hashem.
However, Hacham Ovadia Yosef, as well as Hacham Benzion Abba Shaul (Israel, 1924-1998) in his work Or Le’sion (vol. 2, 13:8), disagreed. In their view, one may recite "Asher Natan" as long as he has yet to reach the word "La’ad" in the next Beracha of Birkat Ha’mazon. After all, the first words of the next Beracha – "Baruch Ata Hashem Elokenu Melech Ha’olam" – are the same words that begin the Beracha of "Asher Natan." The first word that is not shared by the Beracha of "Asher Natan" is "La’ad." And therefore, according to these Poskim, if one recited "Baruch Ata Hashem Elokenu Melech Ha’olam" and then realized that he had forgotten "Reseh" or "Ya’aleh Ve’yabo," he can simply continue by reciting "Asher Natan" instead of "La’ad." He then begins the next Beracha of Birkat Ha’mazon anew after completing the Beracha of "Asher Natan."
Summary: If one realized after completing the Beracha of "Boneh Yerushalayim" in Birkat Ha’mazon on Shabbat that he had forgotten to recite "Reseh" (or "Ya’aleh Ve’yabo" on Yom Tob), then he recites in lieu of "Reseh" (or "Ya’aleh Ve’yabo") the special Beracha of "Asher Natan" which is printed in some editions of the Siddur. He can recite this Beracha even if he began the next Beracha, as long as he had yet to reach the word "La’ad."
