In a bowl, toss the sliced chicken with the salt and pepper, garlic and cornstarch mixture. Set aside. Heat 3 tablespoons of the corn oil in a wok and add the mushrooms, cabbage and sugar. Stir-fry for 2 minutes, then cover and cook for 5 minutes. Remove from the wok. Heat the remaining 2 tablespoons of corn oil in the wok and stir-fry the chicken for 2 minutes over high heat. Add the soy sauce and mix well. Cover and cook for about 6 minutes, or until the chicken is cooked through. Add the cooked vegetables and the scallions. Toss everything together for about 1 minute. Serve hot with steamed rice.