Slice the zucchini and onion thinly.
Heat some cooking oil in a pan and sauté the onion and zucchini.
When the zucchini is half cooked, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients over rice.
Sprinkle seaweed flakes and sesame seeds on top, if desired.