Soak mushrooms for 30 minutes in enough warm water to cover. Rinse well; squeeze. Chop finely, discarding stems. Combine the 1/4 cup water, oyster sauce, cornstarch, sugar, soy sauce, and pepper; set aside. Pour oil into a wok or large skillet. Preheat over medium-high heat. Stir-fry ginger and onions in hot oil for 15 seconds. Remove. Add pork to wok. Stir-fry for 2 to 3 minutes or until no pink remains. Push from center. Stir oyster sauce mixture. Add to center of wok. Cook and stir until thickened and bubbly. Add mushrooms, onion mixture, and bamboo shoots. Stir all ingredients together. Cook and stir 1 minute more. Makes about 1-1/2 cups filling (12 servings).