Soak lemon gras sin warm water for 2 hours, chop fine. Cover ginger with vinegar and set aside. Season chicken with fish sauce, salt and pepper. Add lemon grass and mix well. Combine in small bowl, cornstarch, water and sugar. Heat oil in frying pan and saute garlic and onion in it. Add chicken mixture and scallion, fry 5 mintues then turn and fry another 5. Add ginger and the cornstarch mixture. Stir well, cover cook 10 minutes. Serve over rice.