Add the sugar and mirin to the cooked sushi rice. Leave to cool.
In a bowl, combine the drained tuna, sweet chili sauce, and finely diced onions.
Place a nori sheet, rough side up, on a sushi mat. Spread the rice over the nori, pressing firmly and leaving a 5cm gap on the side furthest from you.
Place the tuna on the closest edge. Using the mat to get started, roll the sushi tightly to form a roll. Moisten the 5cm gap at the end of the nori with water to help seal the roll. Chill until needed.
Just before serving, slice the rolls into 1/2 inch (1 1/2cm) slices.Option: You can add wasabi into the sushi before rolling, or serve it on the side and let people decide how much they want.