Prepare onion and canned tuna, slice the onion thinly.
Heat oil in a pan and sauté the onion.
When the onion becomes translucent, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper to taste.
Place the sautéed onion and tuna mixture over rice.
Optionally, top with chopped green onions, seaweed flakes, etc.