Put the rice in a heavy-based saucepan with the measurement water. Bring slowly to the boil, then reduce the heat and simmer, half-covered, for 5–8 minutes, or until all the water has been absorbed. Cover completely and cook very gently for a further 5 minutes, or until the rice is very tender and sticky. Turn into a bowl and leave to cool.
Stir the spring onions, vinegar, sugar, ginger and sesame seeds into the rice.
Use scissors to cut the nori sheets into 6 cm (2½ inch) squares. Dampen your hands and mould the rice into little ovals. Arrange the rice ovals diagonally over the nori squares.
Bring the pointed ends on opposite sides of the nori over the rice and arrange a piece of fish or a prawn on top. Arrange on a serving platter and serve with a small bowl of soy sauce for dipping.