1. Pull skin from chicken breasts; slice meat thin. Trim mushrooms and slice. Drain bamboo shoots and slice, if needed; trim green onions and celery; slice both thin. Halve red pepper, seed, and dice. (If using pimientos, drain and cut into strips.)
2. Saute chicken quickly in vegetable oil in a large frying pan 3 minutes, or until chicken turns white.
3. Stir in mushrooms, bamboo shoots, green onions, celery and red pepper; saute 2 minutes. Stir in sherry and soy sauce; cover. Cook 2 minutes, or until vegetables are crisply tender.
4. Blend cornstarch into chicken broth until smooth in a small bowl; stir into frying pan. Cook, stirring constantly, until sauce thickens and boils for 3 minutes.
5. While chicken mixture cooks, prepare rice with boiling water and salt, following label directions. Spoon around edge on a large serving platter; spoon chicken mixture on top. Garnish with a half mushroom and serve with additional soy sauce, if you wish.