Soak the seaweed in cold water for about 10 minutes, then drain.
Heat sesame oil in a pan and stir-fry the seaweed for 1 minute.
Add tuna and stir-fry together for another minute.
Pour in 400ml of water and add oatmeal once it boils.
Simmer over medium-low heat for 5 minutes, stirring occasionally.
Season with salt or soy sauce to taste.
Transfer to a bowl and serve.