Cut the chicken, enoki mushrooms, and onion into bite-sized pieces.
Heat some cooking oil in a pan and stir-fry the chicken.
When the chicken is cooked, add the onion and enoki mushrooms, and stir-fry together.
Season with soy sauce, mirin, and sugar, and stir-fry until the flavors are well absorbed.
Spoon the stir-fried ingredients over rice to serve.