1. Cook the rice until ready. Place in wooden bowl. Using a wooden spoon, quickly mix with rice vinegar, sugar and salt, touching as little as possible. Keep in a rice box at 40°C, the optimum temperature for sushi rice.
2. Cover the top side of a bamboo roll mat with plastic cling film. Lay nori seaweed on one half.
3. Spread rice over the nori until it is covered, around 2 mm high. Sprinkle with sesame seeds.
4. Turn the nori seaweed and rice over, using the mat to help.
5. Cut tuna into six pieces, each 5 cm long and 4 mm thick. Lay them lengthwise, three at a time.
6. Place sliced avocado next to tuna.
7. With both hands, roll the sushi into a log using the mat. Turn it over once and press into an oblong square shape. Remove the mat. Cut into six pieces of similar size.
8. To serve, place a banana leaf in the center of a plate. Arrange the pieces in twos, with two sets on their side, and two upright. Add pickled ginger and wasabi.