Cut the chicken thigh into bite-sized pieces and marinate with salt and sake for about 15 minutes.
In a small pot or pan, combine the 'A' ingredients (water, shiro dashi, soy sauce, mirin, sugar, vinegar) and bring to a boil. Add the onion and chicken, cover and simmer for 5 minutes on medium-low heat.
In a bowl, crack the eggs and lightly beat them, cutting the egg whites about 10 times with a fork.
Pour half of the egg mixture into the pot, cover and cook for 1 minute on medium heat.
Pour the remaining egg on top, cover and cook for another 30 seconds.
Serve the oyakodon over rice and garnish with mitsuba.
