I have been hesitant to put a writeup here because I feel that this topic can be almost as delicate as sashimi itself is. However, after several requests, most persisently from
alex.tan, I've decided to be blunt.
Most people reading this will not have had the opportunity to encounter real sashimi in their lives, even if they order it weekly at a restaurant. This is not because one cannot eat "real sashimi" unless one is in Japan. Although I think it would be difficult to do so (and I shall explain why later), this is not what I mean. I mean that the fish must be absolutely fresh, caught just that morning, and of the highest possible quality. So if one lives inland rather than in a coastal area, one will almost certainly be served fish that has been frozen. This changes the character, texture, and flavour of the flesh entirely.
Frozen fish is certainly fine for such things as ceviche or even a carpaccio. In the former case, the flavour of the lime and garlic used to cook the raw fish is inherently part of the dish. In the latter, fresh extra virgin olive oil and various herbs provide a great deal of the flavour.
Even in the case of sushi, the vinegared rice, the dab of wasabi used to secure the fish to the rice (in the case of nigiri-zushi, the nori seaweed used in maki), are all components of the dish. The frozen salmon or yellowtail or tuna that is inevitably served as sushi in most restaurants is helped along in this way.
However with sashimi, the fish is nakedly alone as well as raw.
Sashimi can be made with a vast array of fish from tuna to seabream and even cuttlefish. The various delicate flavours are brought forth primarily by the artistry of the chef. Whereas children can be taught to roll out a presentable cucumber-maki, a chef who presents sashimi must have many many years of training, experience, and a particular talent. Finding a restaurant with such a chef is the other matter that makes eating real sashimi difficult.
The crux is in the slicing of the fish. The most common way of slicing sashimi is called "hirazukuri". This consists of slicing it flat, about one centimeter in thickness and several centimeters long. But then there is "kakuzukuri", which is slicing into cubes. "Kawazukuri", slicing yet retaining the skin. "Ikezukuri", which is bit dreadful: slicing of a living fish. And it is through the nuances of methods like these that the sashimi releases its full depth and richness.
Sashimi is served along with ocha (green tea) on a platter with arrangements of garnishes such as shredded daikon, seaweed, shiso leaves, gari and various pickles. Plain rice and perhaps miso soup would be served alongside. Saké would be served before and/or after the meal, but not during.
If you are living in a coastal area, please enjoy sashimi as often as you can. If not, you are probably best to have sushi instead. Always feel free to ask the chef which fish is freshest and how fresh it is.
And of course you can make "sashimi" with frozen salmon or whatever is available. I even call some dishes made with tomatoes and mushrooms "sashimi". But real sashimi does not have quotation marks around it. It is naked and on its own.
I certainly hope I have not hurt anyone's feelings with this writeup. alex.tan said that the truth doesn't hurt. My intention was only to say what I know about sashimi.