Add water to 25g of oatmeal just enough to slightly cover it, add 50g of tuna, microwave for 1 minute and 30 seconds.
Prepare the oatmeal and onion.
Finely chop the onion.
Add 400ml of water, oatmeal, and onion to a pot and bring to a boil.
When the oatmeal is cooked, add tuna and salt, and stir well.
Simmer over medium-low heat for 3 to 5 minutes.
Top with pepper or scallions to taste and serve.