A person who eats 2 oz. or more of bread in the Sukka on Sukkot must recite the Beracha of "Lesheb Ba’sukka" before he begins eating. If a person forgot to recite this Beracha before he started his meal, does he recite the Beracha afterward?
The Mishna Berura (commentary by Rav Yisrael Meir Kagan, 1839-1933) rules that a person who remembers during his meal that he had forgotten to recite the Beracha of "Lesheb Ba’sukka" should recite the Beracha at that point. Preferably, he should ensure to eat at least another ounce of bread or other food after reciting the Beracha, so that the Beracha will be considered as having been recited during, rather than after, the meal. However, the Mishna Berura adds, even if a person remembered only after he finished the entire meal, he may nevertheless recite the Beracha at that point. This applies even if the person remembered to recite the Beracha only after he recited Birkat Ha’mazon. The Mishna Berura explains that when a person remains in the Sukka after the meal, the time spent in the Sukka may be seen as an extension of the meal, and it is therefore not too late to recite the Beracha of "Lesheb Ba’Sukka." Since we still consider the meal in progress, the individual may still recite the Beracha.
However, this is on condition that the person had not left the Sukka before he remembered to recite the Beracha. If a person left the Sukka – even temporarily – after the meal, such as to use the restroom, then his departure from the Sukka constitutes the end of the meal. He therefore cannot recite the Beracha of "Lesheb Ba’Sukka" when he returns, since the meal is no longer considered in progress.
Summary: If a person forgot to recite the Beracha of "Lesheb Ba’Sukka" at the beginning of the meal, he may recite it during the meal, at the point when he remembers. Preferably, he should eat another ounce of food after reciting the Beracha. Nevertheless, even if he remembered to recite the Beracha only after he completed the meal and recited Birkat Ha’mazon, he may still recite the Beracha, provided that he had not left the Sukka since the meal ended.
