Mix the rice vinegar with two teaspoons of sugar and a dash of salt. Stir thoroughly and set aside
Put the rice in a colander and wash thoroughly under cold running water. Leave to drain for 30 minutes. Place in a saucepan with the water. Bring to the boil, while stirring, then reduce to a low simmer. Cover and cook without stirring for 10 minutes. Remove from the heat and set aside for a further 10 minutes. Transfer to a non-metal bowl and add the rice vinegar seasoning. Cut the seasoning through the rice with a wooden spoon, for about two minutes. Set the rice aside and allow to cool to room temperature.
Slice the tuna into 1cm strips and place in a shallow dish. Mix the soy sauce, sake, chilli sauce and finely chopped spring onions in a bowl. Pour the mixture over the tuna and stir well to coat. Cover and marinate for 30 minutes. Divide into six portions.
Place half a sheet of nori, rough side up, on a rolling mat with the long edge towards you. Top with one portion of sushi rice spread in a thin layer. Leave about 2cm of bare nori at the far edge. Place one portion of tuna strips in a line along the middle of the rice.
Lift the near edge of the mat and roll the sushi away from you, pressing in the filling as you go. A little water may be needed to seal the far edge.
Repeat the process to make 6 rolls. Slice each roll into even pieces using a clean wet knife.
«The best cut of tuna for this recipe is the dark red akami variety.»