Cut the chicken into bite-sized pieces and put it in a bowl. Add the sherry, cornstarch and egg white. Stir through to coat evenly. Leave to marinade for about 30 minutes.
Heat a wok and add the oil. Fry the garlic and, as soon as it starts to go brown, toss in the chopped scallions and stir fry them for about half a minute.
Add the chicken and cook for about 4 minutes, stirring and tossing all the while. Remove with a slotted spoon and set aside.
Add the mushrooms to the wok and cook them for 5 minutes or until cooked through, stirring and tossing as before. Stir in the hoisin sauce, then return the chicken and toss all the ingredients together until thoroughly heated through..
Serve at once with boiled rice and a green vegetable.