Soak the seaweed in cold water for 10 minutes, then drain.
Drain the oil from the tuna using a sieve.
Add 400ml of water and seaweed to a pot and bring to a boil.
Once the seaweed is soft, add oatmeal and cook over medium heat for 5 minutes.
Add tuna and salt, then simmer for another 2 minutes.
Season with pepper to taste if desired.