Prepare oatmeal, canned tuna, and onion.
Finely chop the onion.
Heat sesame oil in a pot and sauté the onion.
When the onion becomes translucent, add water and bring to a boil.
Once boiling, add oatmeal and tuna.
Cook on low heat for 3–5 minutes, stirring occasionally.
Taste and season with salt as needed.
Serve in a bowl and top with seaweed or green onion as desired.