In a shallow 1 1/2-quart microwaveable dish, combine the sherry, soy sauce, sugar substitute brown sugar syrup, ginger, garlic, and five-spice powder. Add the chicken and turn to coat. Cover with vented plastic wrap and microwave for 5 minutes on high power. Turn the chicken and cook for 5 minutes longer.
Place the chicken on a serving platter. Do not discard the juices in the dish.
In a cup, dissolve the cornstarch in the water and stir into the juices in the dish. Cover with vented plastic wrap and microwave on high power for 1 1/2 minutes. Stir and pour over the chicken. Sprinkle the scallions over the top for garnish before serving.