Cut the chicken breast into very thin slices.
The best way to do this is to place the meat on a board then, holding it firmly with one hand, slice it with the other.
Blend the cornflour with the water and work half into the pieces of chicken.
Set aside for a minute or so.
Heat the oil till very hot.
Fry the chicken pieces in it for 1 1/2-2 minutes, stirring and tossing them as they cook.
Add the stock, seasonings, sherry, remaining cornflour mixture and oyster sauce.
Bring to a good boil and simmer for 1 1/2 minutes.
Turn into a heated dish and garnish with parsley.