To make the seasoning, place the vinegar, sugar and salt in a saucepan, bring to a boil, reduce to a simmer and stir until the ingredients are dissolved.
Place the rice in a colander and wash thoroughly under cold running water. Leave the rice to soak in a saucepan with the 550ml of water for 30 minutes.
Bring the rice to the boil and then reduce to a low simmer. Cover with a lid and cook without stirring for 10 minutes. Remove from the heat and set aside, covered, for another 10 minutes.
Place the rice in a large bowl. Pour the vinegar seasoning over the rice and mix the seasoning through the rice. Continue turning the rice for a couple of minutes. Cover with a damp cloth and leave to cool to room temperature before serving.
Pick through the crab to remove any shell or cartilage. Cut the avocado in half, remove the stone and thinly slice the flesh. When sliced, squeeze some lime juice onto the avocado.
Cut the nori sheet in half. Place, rough side up on a rolling mat with the long edge towards you. Cover with one portion of sushi rice spread in a thin layer. Leave about 2cm of bare nori at the far edge. Sprinkle on some black sesame seeds and press gently to make sure the seeds are stuck to the rice.
Turn the nori and rice over so as the rice covered side is against the sushi mat and the bare nori strip is closest to you. Place some crab, avocado and a smear of wasabi on the bare nori strip. 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.
«Squeezing lime juice onto the avocado once it is cut will prevent the flesh from discolouring when exposed to the air.»