Soak dried seaweed in cold water for 10 minutes, then squeeze out the excess water.
Slice the onion thinly.
Heat sesame oil in a pot, then stir-fry the soaked seaweed and onion for 2 minutes.
Drain the oil from the canned tuna and add the tuna to the pot. Stir-fry with the seaweed and onion for 1 minute.
Add 500ml of water and bring to a boil.
Season with soup soy sauce and salt to taste.
Simmer on low heat for 10 minutes, then add chopped green onion to finish.