As with all seafood, it's important that the sea urchin is fresh. Smell is always the best indicator. They are are usually sold alive, and you can sometimes see the spines gently moving.
If you're dealing with a small sea urchin, use a good pair of kitchen scissors to cut round the 'equator'. When dealing with a larger sea urchin, it's most common to puncture one end, and then cut a small circle of shell out of the bottom, so that the majority of the shell stays intact.
Drain any dark-coloured liquid and then use a spoon to scoop out any other brown-black substance inside the shell. At this point, you can see how methodical sea urchin preparation is. Sea urchins are five-fold symmetrical creatures, with five yellow-orange gonads running up the inner wall. These look like little tongues, both in shape and also in texture. Rinse out the shell at this point, if needed, to have a clearer view.
Use the spoon to gently scrape the orange gonads away from the edge of the shell and then rinse them in fresh, or salt water. If they aren't being served immediately, then make sure that they remain chilled and use as soon as possible.
In terms of flavour pairing, think of sea urchin as you would think of caviar. They both have a delicate flavour with salty-oceanic notes. As with caviar, sea urchin is often simply spread on a biscuit or a piece of toast to showcase the flavours without any distraction. In Japanese cuisine, it is common to use sea urchin to top plain nigiri sushi or stirred into plain noodles. And in Italy, sea urchin is often stirred through a bowl of tagliatelle.
Although sea urchin is an exciting ingredient to experiment with, it is possibly at its best when enjoyed in its purest form – eaten straight from of the shell. In many countries it's traditional to pair sea urchin only with a sip of the local liquor, be that Japanese sake or Greek ouzo.