At a non-kosher restaurant, is thinly-sliced raw fish kosher?
Let's assume one sees the preparer clean the knife and cut from the whole fish.
Also, let's assume this raw fish is sashimi — minus the usual rice, and minus the usual Asian white radish.
(I'm not asking whether or not it's permissible to eat it: if I were, we'd have to consider maris ayin. I'm only asking whether or not it's kosher.)
If the fish is kosher (i.e. you can identify it as a kosher species in an acceptable way, e.g. you see the scales on the whole fish, or for skinless salmon by the red color of its flesh which is a sign of kashrus), and everything is cold and clean (fish and utensils), then the fish can be eaten l'chatchila. This is based on the Shach in Yoreh Deah 91 (#3), as well as the discussion in the Aruch HaShulchan in Yoreh Deah 91:6, where it is clear that one may use cold, non-kosher utensils to eat kosher food, as long as it is done only occasionally.
Since nothing is hot, the question of ben yomo or eino ben yomo (whether it was used in the last 24 hours) does not even matter. And as long as you are just eating the fish, then the issue of charif does not matter either. (If however, a non-kosher knife was used to cut a charif food, like an onion, then you could not eat the onion. But if it subsequently cut fish, then the fish is still fine, because fish is not considered a davar charif).
So the bottom line is that the only issue is whether the fish is kosher and cold and the knife is clean. If all you are eating is the raw fish, then there are no issues of charif or kli ben yomo.
(For more on the topic of identifying skinless salmon or other salmanoid fish based on its distinctive red color, see: http://www.oukosher.org/index.php/common/article/the_kashrus_of_skinless_salmon1/ . For white-fleshed fish, the topic becomes more complicated, whether one is expert enough to identify a fish based on its flesh and whether that expertise is valid)
Normally it is not.
When using a knife to cut kosher food there are two requirements.
Normally, a person might want to argue that the Fish is cold and so none of these are really issues. However, Sushi is normally NOT halachically cold.
There are two types of "heat" when dealing with kashrut.
Under spicy "heat", the common example is an Onion. If the knife cuts an onion, and before cleaning off the knife cuts squid or cooked eel, or a number of other unkosher foods found at a place that serves Sashimi, or more likely, even cuts them at the same time, the knife is no longer kosher. In a restaurant that serves sashimi there is a good chance that Wasabi, Soy Sauce, Radish, Rice wine Vinegar, Ginger, or many other vegetables, all of which count as "hot" will have treifed up the knife less than 24 hours before your sashimi was cut.
I have heard of people who have their own knife which they have given to a sushi chef in places where kosher agencies are not around.
Edit: I see the need to quote a source here so:
Furthermore, davar charif is not limited to imparting tastes into food; it can also impart a taste into a utensil. One scenario would be when a mixture of meat and sharp spices is being chopped with a blade. The sharpness of the spices combined with the pressure of the blade will cause the meat taste to become absorbed into the chopper.[16]
This stringency is also relevant to cutting boards. The Chochmas Odom[17] discusses cutting a piece of salty herring (a davar charif) on a non-kosher plate. He states that the combination of the sharp taste and the pressure from the blade will draw absorbed issur out from the plate and impart it into the fish. Similarly, Rav Shlomo Kluger[18] writes that if an onion was cut with a fleishig knife on top of a milchig plate, both the onion and the knife will absorb a combination of meat and milk tastes, thus rendering them both non-kosher.[19]
