Trim the root end off the enoki mushrooms, wash under running water, and cut into bite-sized pieces.
Julienne the onion.
Heat cooking oil in a pan and sauté the onion.
When the onion turns translucent, add the enoki mushrooms and stir-fry together.
Add fish sauce and soy sauce, and continue to stir-fry.
Add tuna and mix well while sautéing.
Place the stir-fried mixture on top of steamed rice.
Finish with chopped scallions and a sprinkle of sesame seeds.