Uni is sea urchin roe. When fresh, it is oily and rich that is not firm but rather paste like.

For nigiri sushi, wrap a large strip of nori that covers the sides of the rice ball and protrudes around the top to form a cup (the nori should be 1 by 7 inches). Fill the formed cup with a piece of uni, careful not to let it spill over. Serve immediately as the nori will wilt and turn soggy.