By Rabbi Shimon Anshin shlit”a
In order to know when “Mezonos” must be eaten in a Sukkah, and when a blessing is required, we must differentiate between eating a bread-like food such as cake or a cooked dish whose blessing is Mezonos (for instance, kugel), as well as the amount eaten, how it is being eaten, and why, as we will explain.
 When eating a bread-like food such as cake in an amount more than a k‟beitza (the size of an egg), in a casual manner on Chol HaMoed, Shabbos or Yom Tov not after Kiddush:
[a] It should be eaten in a Sukkah, [and it may be that one who is lenient and eats it outside a Sukkah has on whom to rely]. Still, the blessing, “Leyshev BaSukkah” should not be recited.
[b] The designation of a “casual manner” in this context is that he is just snacking – meaning he is just enjoying a snack and not eating it to satiate his hunger. Or, even if he is hungry, if he is eating with the intention of eating a small amount in order to hold off his hunger until the mealtime, as the Rambam details in Perush HaMishnayos, Sukkah 2:5.
[c] However, if he is planning on staying in the Sukkah for a significant amount of time after eating, then he can recite the blessing, having in mind both the eating as well as the sitting in the Sukkah. This is because in actual fact, we should be reciting a blessing on just spending time in the Sukkah, as we will explain.
There are those who hold that in such a situation one should first recite “Leyshev BaSukkah” and then Mezonos.
[d] I have not yet found a clear designation of “a significant amount of time”. It would seem that it refers to both quality and quantity. This would
mean spending a slightly extended period of time in the Sukkah, as well as actually making oneself comfortable, and not being in a hurry to get out – i.e. spending the time there just in order to “fulfill the requirement” of spending time. Therefore, just a couple of minutes would not be enough. However, if one studies a little bit, or reads a book for fifteen minutes, it would seem that that is enough to be considered “spending time” in the Sukkah and would already require a bracha.
[e] All this is in one own‟s Sukkah. But when one goes to someone else‟s Sukkah, it would seem that any visit, even just for a few minutes, i.e., for a Yom Tov visit, or a Shalom Zachor, would be already considered spending time [this is in conjunction with the opinion that requires a blessing upon using someone else‟s Sukkah even when not spending time there at all.]
 When partaking in a proper meal, during Chol HaMoed:
[a] One is obligated to eat in a Sukkah and to recite “Leyshev BaSukkah”
[b] The designation “proper meal” is for example, to eat cake or pastry in the morning together with a cup of coffee; or without coffee, but instead of breakfast; or to eat together with other people [all these would render the meal to not be any less proper than eating a cooked item which would be considered a proper meal according to the Shulchan Aruch.]
[c] A thoroughly proper meal, which would mean to eat more than three or four beitzim, or entirely with the intention of eating a meal, would require a hamotzi, Birchas HaMazon and would require a Sukkah and a “leyshev BaSukkah.” It would seem that even in such a case, hamotzi and Birchas HaMazon would not be required, still, in regard to Sukkah, he would still be obligated in Sukkah and “leyshev BaSukkah”.
 Eating “Mezonos” on Shabbos and Yom Tov after Kiddush, in order to fulfill the obligation of Kiddush being in the same place where a Seudah will be:
[a] Must be eaten in a Sukkah, and a bracha (“Leyshev BaSukkah”) may be recited even if he‟s not staying there a while, and even if the amount eaten is a little bit more than a beitzah.
[b] However, if not eating for this reason, then the Halachos of Chol HaMoed apply.
 All this is when eating cake, but when eating a cooked dish made out of any of the 5 grains, i.e., kugel, then the halacha is as follows:
[a] More than a beitzah, in a casual fashion- should be eaten in a Sukkah, but without a Bracha “Leyshev BaSukkah”, [and even if one chooses to be lenient and not eat in the Sukkah, he has on whom to rely.]
[b] More than a beitzah, together with other people – requires a Sukkah with a beracha “Leyshev BaSukkah”.
[c] More than a beitzah, and is eating it as a proper meal – would require more research to establish whether or not a blessing is needed, especially Shabbos after Kiddush.
[d] Someone who eats a considerable amount as a proper meal – requires a Sukkah and a beracha “Leyshev BaSukkah”.
In addition, a cooked item may be eaten in large amounts without requiring HaMotzi or Birchas HaMazon, unlike “mezonos”. Kugel would be in the category of „cooked‟ even though there are those who are of the opinion that since it is also baked it falls under the category of mezonos.
As this is a translation of the original Hebrew, if you unclear on any of the Laws outlined herein in any way whatsoever, please consult with a Posek (Halachic Authority).