Cut the chicken breasts into thin strips or cubes. Bring the stock to a boil. Add the soy sauces, mirin, sake or dry sherry and sugar. Bring back to a boil.
Add the chicken, reduce the heat and simmer for 5 minutes. Stir in the scallions and leave stand for 1 minute.
Lightly stir the eggs in a bowl just enough to break them up. Add the eggs to the pan and leave to stand for a further 2 minutes. Carefully stir once.
Spoon hot rice into individual bowls. Serve immediately.
