Soak the dried seaweed in cold water for 5 minutes, then chop it finely.
Prepare oatmeal, seaweed, and tuna.
Pour 400ml of water into a pot and bring to a boil.
Add the oatmeal, seaweed, and tuna to the boiling water.
Simmer over medium heat for 5-7 minutes, stirring occasionally.
Season with salt.
Serve in a bowl.