Place the rice in a heavy based pan with a lid. Cover with cold water and wash the rice, swishing it with your hands, until the water is cloudy. Drain the water and repeat until the water is almost clear – probably about 5 – 6 changes of water.
Once the water is almost clear drain and add ½ pint water to the pan. Sprinkle in a little salt, put the lid on and bring to the boil. Once the water is boiling turn the heat down to the lowest level and cook for 10 minutes, then remove from the heat and leave to stand, with the lid on for another 10 minutes before lifting the lid.
While the rice cooks, stir together the rice wine vinegar and caster sugar until the sugar dissolves. Fluff the cooked rice and turn out to a large bowl. Cool for a moment before stirring through the sushi vinegar.
Mix together the tuna, mayonnaise and spring onion. Place the nori on a sheet of cling film. With dampened fingers press 1/6 of the rice over two thirds of the nori along the long side leaving about 1cm strip uncovered. The rice must be in a thin layer, or the roll will be too full to close. Press a little of the wasabi in a line down the centre of the rice. Top with a line of the tuna mixture. Dampen the nori that isn’t covered in rice and roll up from the rice side, pressing together.
To cut the sushi into pieces wet a serrated knife. Cut each into 5-6 pieces. Serve with soy sauce to dip.