Saute kimchi in kimchi juice over medium heat for 2 minutes. Add tuna and stir to continue sauteing until kimchi is somewhat soft and the mixture seems a little dry.
Add green onion and heat through. Add sesame oil and stir. Set aside to cool a little.
Season rice with a couple of pinches of salt.
To make kimchi tuna onigiri
In a rice patty presser, fill rice to halfway up. Place a tablespoon of kimchi tuna filling over the rice leaving edges and corners untouched. Fill more rice on top to cover to the rim of the presser. Cover the lid and press down. Uncover and release the patty by pushing the bottom of the press.
Cut a sheet of plain roasted seaweed into 8 strips. Wrap the bottom of the rice patties with the seaweed strips.
Dip one corner of rice patties in the roasted sesame seeds to coat.
To make kimchi seaweed rice balls
Mix kimchi tuna filling and 1 tablespoon sesame seeds with rice. Roll the mixture to make small balls, about the size of golf ball.
Crush seasoned roasted seaweeds in a plastic bags until they get crushed into small pieces. Roll the rice balls to coat with seaweeds.