Cut woody stems from broccoli. Peel stems. Cut stems diagonally into slices. Cut broccoli tops into flowerettes. Set aside. Cut onion into wedges. Cut celery diagonally into 1/4 inch thick slices. Rinse bean sprouts and drain. Finely chop garlic. Set aside.Combine cornstarch, water, oyster sauce, salt, and sugar in a small bowl; stir until smooth. Set aside.Heat the wok over high heat about 1 minute or until hot. Drizzle oil into the wok and heat 30 seconds. Add broccoli stems, onion, and celery. Stir fry 2 to 3 minutes or until vegetables are crisp tender. Add broccoli flowerettes, garlic, and broth. Cover the wok and cook about 3 minutes or until broccoli is crisp tender.Stir cornstarch mixture until smooth. Add to the wok. Cook and stir until sauce boils and thickens.Add bean sprouts and pimiento to the wok; stir fry just until heated through.Transfer to a serving dish.