Prepare onions and canned tuna, then thinly slice the onion.
Heat oil in a pan and sauté the onions.
When the onions are half-cooked, add the tuna and stir-fry together.
Season with soy sauce and black pepper to taste.
Place the stir-fried onion and tuna over rice.
Optionally, add seaweed flakes or a fried egg as a topping.