Prepare onion and tuna, and finely chop them.
Heat cooking oil in a pan and stir-fry the onion.
When the onion becomes translucent, add tuna and stir-fry together.
Season with soy sauce and pepper.
Add rice and stir-fry, mixing all the ingredients well.
Finally, drizzle sesame oil and mix once more.
Serve the finished onion tuna fried rice on a plate.