Excerpt from a (negative) Yelp review of a local sushi shop:
"They slice a lot of the fish for the nigiri in advance, at the beginning of the service. Fish should be sliced as the nigiris are made."
Wrong. Wrong. So wrong. And yet this is a common misconception. I've already covered this topic in my Sushi Zanmai review, but this subject probably warrants its own post.
Sushi-meshi (aka shari or sushi rice) is best when served at body temperature. Most neta (aka tane, or sushi topping) taste best when served at room temperature (this is a general rule but not the case for all neta). You NEVER EVER want to be served cold sushi rice. And typically, you also do not want to be served cold neta.
Why is most neta best served at room temperature? Because subtle flavors are too muted when cold, and the texture of the neta will be affected as well. Allowing the topping to slowly rise to ambient temperature insures that both flavor and texture are optimal. And cold neta can be a harsh and unpleasant contrast to body-temperature shari.
The itamae should take an ingredient out of the cold box, then slice it about 15 or 20 minutes before serving (again, this can vary). The sliced neta then sits on the counter (as shown in the photo above) for a while until it reaches its optimal serving temperature.
If the chef was to take an ingredient out of the cold box, slice it, then immediately serve it, the neta would be much too cold to taste good.