Soak the seaweed in cold water and chop it finely.
Finely chop the onion.
Heat sesame oil in a pan and sauté the onion.
Add drained tuna and sauté together.
Add the seaweed and continue sautéing.
Pour in 400ml of water and bring to a boil.
Add the oatmeal and simmer for 3-5 more minutes while stirring.
Season with salt.