Place the rice in a bowl and add cold water. Swish around with your hand, drain the water and repeat until the water runs clear.
Place the rice, a small piece of kombu and 150 ml water in an unperforated cooking container and cook for 18 min at 100°C in an electric steamer. Stir occasionally.
Heat the rice vinegar and stir in a pinch of salt and a pinch of sugar. Transfer the rice to a bowl and remove the kombu. Slice through the rice using a rice paddle or a wooden spoon and slowly drizzle the seasoning into it.
As you do this fan the rice (use a hand fan, your hand or a piece of cardboard) so that the rice quickly cools to room temperature and becomes shiny.
Split the rice into 8 portions and, with wet hands, shape each portion into a tube. Place the rice at the top of a halved nori sheet and press flat a little. Add some wasabi paste, place the fish and negi on top and roll into a cone-shaped temaki sushi. Serve with soy sauce.