1. Rinse chicken and pat dry. Cut in half and pound slightly to thin. Cut crosswise into thin strips (about 1/4-inch thick).
2. Whisk together garlic, soy sauce, 2 teaspoons cornstarch and the ginger in a large bowl, for marinade. Add chicken, turning to coat, cover and chill 45 minutes, stirring occasionally.
3. Whisk together juice and remaining 2 teaspoons cornstarch in a small bowl until dissolved.
4. Heat 2 teaspoons oil in a heavy skillet over high heat, until hot but not smoking. Remove chicken from marinade (discard marinade) and add to skillet; saute, turning occasionally, until golden, about 5 minutes. Transfer to a bowl. Add remaining 2 teaspoons oil, the mushrooms, carrot, and celery to same skillet; saute over moderately high heat until tender, about 5 minutes. Return chicken to skillet; add juice-cornstarch mixture and bring to a boil. Stir until sauce thickens, about 3 minutes.
5. Serve over rice and garnish with scallion, if desired. Makes 8 servings.