Cut the pork evenly into thin slices, then into slivers. Add the marinade ingredients and mix well.
Bring a pan of water to the boil. Add the noodles and cook for 2 minutes (fresh Shanghai and udon noodles are already half-cooked when you buy them, which is why this doesn't take long). Turn the cooked noodles into a colander and cool under the cold tap. Shake them dry. Drip over 1/2 tablespoon oil and stir in thoroughly to prevent sticking.
Heat 1 tablespoon of the oil in a seasoned wok over a high flame. Add the pork strips and stir-fry swiftly to separate them. When they are just cooked, remove from the wok and set aside. Clean and re-season the wok if necessary, then return it to a high flame with the remaining oil. Add the noodles and stir-fry until piping hot, adding both soy sauces and seasoning with salt and pepper. Add the bok choy or spinach and continue to stir-fry briefly until wilted. Finally, stir in the pork. Serve.
"Land of Fish and Rice" by Fuchsia Dunlop © W.W. Norton & Company 2016. Provided courtesy of Fuchsia Dunlop. All rights reserved.