I am not sure what 上 means in this sentence. I know that 上 means above or on, but I think the meaning here is different because it does not mention anything above or on the sushi.
僕はいつもここでは握り寿司の上を頼むんだ。
Does it mean:
I always ask for the top hand-rolled sushi here.
i.e. best hand-rolled sushi?
Yes, 上 in a restaurant menu is read じょう, and it means "deluxe", "premium" or "high grade". But 上 is often not the highest ranked menu, because restaurants commonly offer 特上 menus, too.
特上【とくじょう】 (lit. super deluxe) ＞ 上【じょう】 (lit. deluxe) ＞ 並【なみ】 (lit. normal)
So practically, 上 is often considered as "middle grade".
Another traditional way to express the rank of food is 松【まつ】 ＞ 竹【たけ】 ＞ 梅【うめ】, which are frequently used in bento shops, sushi restaurants and unagi restaurants.
Reference:
