Heat half the oil in a frying pan over high heat and cook the chicken until golden and cooked through. Set aside. Add the additional oil and the sliced onions; cook over low heat until tender but not browned. Add the soy, water, and mirin and bring to the boil. Return the chicken to the pan.
Lower heat to low and pour in the eggs. Using a wooden spoon, stir it around once to distribute the mixture a little. Cover and cook for 5 or so minutes, or until the egg is set.
Add rice to bowl and top with the chicken. Garnish with spring onion and pickled ginger.