There is a concept in the area of blessings over foods known as Ikar and Tafel, or primary and secondary foods. In other words, foods have different levels of Halachic importance and in some cases, reciting a blessing over a food which is considered more important can exempt a different type of food which is eatenalong with it. In such a case, the former food would be considered Ikar, or primary, and the exempted food is secondary to it, or Tafel. One example is a bowl of cereal, in which the cereal’s blessing is “HaAdama” and the milk is “Shehakol”, one would only have to recite a blessing on the Ikar-the cereal-and that would exempt the milk. Another example is a rice dish with meat mixed into it. The Shulhan Aruch (Orah Haim 208:7) rules that whenever two foods are mixed together, that which is in the majority is considered Ikar. As such, it would seem as though the rice would be considered Ikar. Nevertheless, the Shulhan Aruch refers to food which are very similar (such as rice flour and millet flour) and in the case of the rice dish, the rice and meat are unique entities. Furthermore, the meat is the main satiating component of the mixture, whereas the rice acts as a filler, and therefore, the meat would be considered Ikar and a “Shehakol” blessing would be sufficient for the entire dish.
One contemporary application of this concept is sushi. On one hand, the majority of a serving of sushi is rice, suggesting that the blessing should be “Mezonot”. On the other hand, many people eat sushi particularly for the fish meat contained within, and thus “Shehakol” would need to be recited instead. If one’s intention in eating the sushi was for the rice, then one would recite“Mezonot”.
Another example is in a vegetable-containing couscous dish. If the couscousand vegetables are mixed together, then “Mezonot” should be recited because the Halacha is that a grain-based food is always Ikar. However, there is disagreement among the Moroccan Poskim when the couscous and the vegetable are separate. Rabbi Yosef Berdugo (Shufre DeYosef, § 20) rules that since reciting “Mezonot” may possibly exempt the vegetables even though the foods are separate, one should recite “HaAdama” first on the vegetables and then “Mezonot” on the couscous. Rabbi Yosef Messas (Otzer HaMichtavim, vol. III, § 1781), however, says that the vegetables and couscous are unquestionably considered to be separate and that one could recite “Mezonot” first on the couscous.
Summary: One should recite a blessing over whichever component of sushi that one specifically desires. When couscous and vegetables are eaten together but are not mixed, one may recite “Mezonot” or “HaAdama” in any order.