Drain the oil from the canned tuna and put it in a bowl.
Finely chop the onion, soak it in cold water, then drain.
Mix the tuna with mayonnaise, chopped onion, salt, and pepper.
Add a little salt to the rice and let it cool.
Take a handful of rice, flatten it, and place the tuna filling in the center.
Wrap the rice around the filling and shape it into a ball.
Wrap with seaweed to finish.