If one forgot to recite "Reseh" in Birkat Ha’mazon after one of the first two Shabbat meals, and he realized his mistake after reciting "Boneh Yerushalayim," before beginning the next Beracha, he recites at that point a special Beracha that was instituted in lieu of "Reseh." This Beracha – "Asher Natan Shabbatot…" – is found in many Siddurim, and it was formulated for precisely this case, where one realized immediately after completing the Beracha of "Boneh Yerushalayim" that he had forgotten to recite "Reseh" before "Ve’tibneh Yerushalayim."
The question arises as to whether this Beracha should be recited by one who cannot remember at that point whether he recited "Reseh." On the one hand, we might say that this Beracha should not be recited in such a case, because if the individual had, in fact, recited "Reseh," this Beracha will have then been recited in vain. On the other hand, if he had not recited "Reseh," then his recitation of the next Beracha of Birkat Ha’mazon will be in vain, since he cannot fulfill the obligation of Birkat Ha’mazon without "Reseh."
Hacham Ovadia Yosef resolves this conundrum by adding another factor – the likelihood that the individual omitted "Reseh," reciting Birkat Ha’mazon the way he normally does during the week. Given that it is more likely that he omitted "Reseh" and recited Birkat Ha’mazon the way he does on weekdays, he should recite the Beracha of "Asher Natan Shabbatot" and then continue with the next Beracha of Birkat Ha’mazon, as he would if he knew for certain that he had mistakenly omitted "Reseh."
(It should be noted that if after one completes Birkat Ha’mazon he cannot remember whether or not he recited "Reseh," then, according to the ruling of Hacham Ovadia Yosef, he does not repeat Birkat Ha’mazon. Although it is more likely that the person omitted "Reseh," nevertheless, since the recitation of the entire Birkat Ha’mazon is at stake, we introduce the factor of "Emat Ha’Shabbat" – the awareness of the sanctity of Shabbat – which makes it likely that the individual did, in fact, recite "Reseh.")
Summary: If a person completed the Beracha of "Boneh Yerushalayim" in Birkat Ha’mazon after one of the first two Shabbat meals, and before proceeding to the next Beracha he realized he did not recite "Reseh," or cannot remember whether he recited "Reseh," he should recite at that point the Beracha of "Asher Natan Shabbatot" which is printed in many Siddurim.