Mix the tuna salad ingredients in a small bowl. Season the steamed rice with salt.
Wet hands with water. Take about 3/4 cup rice in one hand, press and form into a triangle shape. Make a well in the middle of the rice ball and put in tuna filling. Close the well with 1/4 cup rice. Reshape into a triangle.
Wrap with a sheet of roasted seaweed. Repeat to make another three Onigiri.
Notes
We adjusted the tuna quantity to match a standard 5 oz can, which is the most commonly available size. This recipe makes enough tuna salad filling for 6–8 onigiri (rice balls). Any leftover filling can be stored in the refrigerator for a few days.