If desired, partially freeze meat for easier slicing. Trim fat from meat. Thinly slice pork across the grain into bite-size strips. For sauce, in a bowl stir together water, soy sauce, cornstarch, sherry, bouillon granules, and pepper. Set aside.
Pour oil into a wok or 12-inch skillet. (Add more oil as necessary during cooking.) Preheat over medium-high heat. Cook and stir ginger in hot oil for 15 seconds. Add celery; cook and stir for 2 minutes. Add mushrooms, fresh bean sprouts (if using), and green onions. Cook and stir for 1 to 2 minutes or until celery is crisp-tender. Remove vegetables from wok.
Add half of the pork to hot wok. Cook and stir for 2 to 3 minutes or until juices run clear. Remove from wok. Repeat with remaining pork. Return all meat to wok. Push from center of wok.
Stir sauce; add to center of wok. Cook and stir until thickened and bubbly. Add cooked vegetables, water chestnuts, and, if using, canned bean sprouts. Cook and stir until heated through. Serve with chow mein noodles.