Rinse chicken thighs. Pat dry and cut into 3/4 inch pieces. Remove any visible fat. Place thigh meat in medium sized bowl. In another bowl, combine beef broth, dry sherry, soy sauce, vinegar, sugar and ginger. Pour enough of the mixture to moisten the thigh meat. Stir well. Marinate 10 minutes. Add cornstarch to remaining broth mixture. Set aside.
In wok or large skillet over high heat, heat oil. Add thigh meat, green onions, and garlic. Stir fry for 3 minutes or until browned. Add green pepper; stir fry one minute longer.
Stir remaining sauce and add to wok. Boil one minute, stirring constantly. Stir in walnuts. Serve immediately over hot rice.