Prepare the enoki mushrooms and tuna by cutting them into bite-sized pieces.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion.
When the onion becomes translucent, add the enoki mushrooms and tuna, and stir-fry.
Season with soy sauce, sugar, and pepper.
Spoon the stir-fried mixture over rice to finish.