Wash the rice under cold water until the water runs clear. Drain well and place in a saucepan with cold water. Cover, bring to the boil and cook for 3 minutes at boiling point. Lower the heat and cook for about 12 minutes. Remove from the heat and leave the rice to rest for 10 minutes.
Dissolve the sugar and the salt in the rice vinegar. Spread the rice on to a large tray or baking dish. Once the rice has cooled slightly, pour over the vinegar mixture and mix gently. Leave to cool to room temperature.
Cut the avocadoes in half, remove the stones and the skin then cut in to thin slices. Cut the tuna in to 10cm long batons.
Place a nori leaf on sushi rolling mat and cover with a thick layer of rice and press down. Sprinkle sesame seeds over the rice then cover with cling film and turn over. Place tuna and avocado on to the nori leaf then fold up to give a square sushi roll.
Serve with soy sauce.
«You can also cook the rice in a rice cooker.»