Finely chop the onion.
Add olive oil to a pan and sauté the onion.
When the onion becomes translucent, add tuna and sauté together.
Dice the tomatoes and add to the pan, cooking until soft.
Add oatmeal and water, stirring well while bringing to a boil.
Once the porridge thickens, season with salt to taste.
Transfer to a bowl and serve.