Bring a large pot of water to boil over night heat. Rinse the noodles under the kitchen faucet to remove any excess cornstarch. Place the bean sprouts in a large colander and set in the kitchen sink. Cook the noodles in the boiling water until they begin to float to the top, about 1 minute. Do not overlook the noodles; you want them to be al dente.
Pour the noodles into the colander with the bean sprouts. Give the colander a good shake and transfer the noodles and bean sprouts to a bowl. Toss them with the oil, soy sauce, oyster sauce, and black pepper to taste. Toss in the char siu and green onions. Serve with a dash of hot mustard.
