Cut the onion, tuna, and other ingredients into bite-sized pieces.
Add oil to a pan and stir-fry the onions first.
When the onions become translucent, add the tuna and continue stir-frying.
Place the sautéed ingredients over the rice.
Add soy sauce or other seasonings to taste.
Top with seaweed flakes, sesame seeds, or other toppings as desired.