1.Drain the tuna and squeeze it firmly to remove excess water - this single step does more than anything else to keep the onigiri from turning soggy - then mix with the mayonnaise and soy sauce.
2.Wet your hands with water, then rub a pinch of salt evenly between your palms.
3.Scoop a portion of warm rice into your palm, press a small hollow in the center, add a spoonful of tuna filling, and mold the rice into a triangle around it, pressing gently - don't over-squeeze, or the onigiri turns dense.
4.Wrap a strip of nori around the base just before serving so it stays crisp.