Prepare onions and canned tuna, and chop them finely.
Heat oil in a pan and sauté the onions.
When the onions become translucent, add the tuna and stir-fry together.
Season with soy sauce, mirin, pepper, and other seasonings.
Place the sautéed onion tuna mixture on top of rice.
Finish with chopped green onions or sesame seeds.