Rinse chicken and pat dry with paper towels. Cut chicken into 1 inch pieces.Heat the wok over high heat about 1 minute or until hot. Drizzle oil into the wok and heat 30 seconds. Add chicken and stir fry about 4 minutes or until chicken is well browned and no longer pink in the center.Reduce heat to low.Add soy sauce, sherry, ginger, sugar, and garlic powder. Cover the wok and cook 5 minutes.Cut lettuce into 1/2 inch wide slices. Rinse, drain and pat dry. Combine broth and cornstarch in a cup; stir until smooth.Increase heat to high. Stir peas into chicken mixture. Cover the wok and cook about 2 minutes or until heated through. Stir cornstarch mixture until smooth. Stir into chicken mixture. Heat until sauce boils and thickens. Add lettuce and stir until wilted.Transfer to a serving dish.Serve with hot cooked rice. Garnish, if desired.