Slice the zucchini into half-moon shapes.
Julienne the onion.
Drain the oil from the tuna using a strainer.
Add cooking oil to a pan and sauté the onion and zucchini.
Once the zucchini is cooked, add the tuna and stir-fry together.
Season with soy sauce and pepper.
Place the sautéed zucchini and tuna mixture on top of rice.