Dunno about authentic, but this got me through freshman year.
6 cups hot cooked rice
1 chicken breast (both sides)
6 beaten eggs
6 large dried Chinese mushrooms
1 large onion
2/3 cups mirin or dry sherry
1/3 cup soy
3.5 T sugar
1/2 cup chicken stock or water
Soak and mushrooms until soft in warm water. Cut into thin strips. Cut chicken into strips and into bite-size pieces. Halve the onion and slice thin-thin. Mix soy, mirin or sherry, sugar, stock and bring to a boil in wok over high. Stir in chicken, mushrooms, and onion. Reduce to medium and cook until chicken is done. Lower heat and stir in eggs. Divide into six and serve over rice.
I make mine much the same as Sam's but my broth is dashi based with some salt, mirin and sugar. We like to add onions at the beginning with the chicken so they're tender. Add eggs right at the end to the hot broth and put a lid on it so they almost completely cook. Runny is a bit good.
I listed oyaku donburi as a comfort food, but never thought of it as having a recipe. is it more than: poached skinless-boneless chicken plus a bit of lightly cooked egg (Japanese style "omelette" or scrambled, served over hot gohan with some of the chicken stock (that had a bit of grated ginger, touch of salt); top with chopped chives or green onion plus a few drops of shoyu? Years ago we had oyaku donburi (and other donburi) for cold winter lunches.