Mix a 200g can tuna with 2 tbsp mayo and 1 tsp wasabi. Place a toasted nori sheet on a board and spread a third of the rice on top in a 1cm layer. Sprinkle over 1 tbsp toasted sesame seeds and pat down. Cover with cling film, then the rolling mat and quickly turn over. Place 1-2 strips of round lettuce across the middle of the nori, top with a third of the tuna mix and thin avocado slices. Roll firmly. Repeat twice more. Cut each roll into 6 pieces. Makes 18 pieces.