Cut off the bottom quarter of each nori sheet. Reserve the small cut-off pieces for another use.
Combine the chopped tuna, spring onion, mayo and chilli sauce.
Centre one sheet of nori on a bamboo sushi mat. Wet your hands. Using your hands, spread a thin layer of rice on the sheet of nori, and press into a thin layer, leaving a 1 to 2cm space at the bottom edge. Sprinkle with sesame seeds. Arrange 1/4 of the tuna mixture in a line across the rice, about 1/3 of the way down from the top of the sheet.
Wet the uncovered edge of the nori. Lift the top end of the mat, and firmly roll it over the ingredients. Roll it forward to make a complete roll. Repeat with remaining ingredients.
Slice the rolls into 2.5cm pieces using a wet knife. Serve straightaway or place in the fridge till serving.