Drain the oil from the canned tuna using a strainer.
Thinly slice the onion.
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 and sugar to taste.
Place the sautéed onion and tuna mixture over cooked rice.
Finish by sprinkling chopped green onion or scallions, and sesame seeds on top.