Pre-cook the sweet potato noodles, following the instructions on the package. Drain and toss in a teaspoon of oil to prevent sticking. Set aside.
Heat 3 tablespoons oil in a wok over high heat until just starting to smoke. Brown the pork belly for about a minute, and then turn the heat down to medium. Remove the pork belly from the wok and set aside. Add the ginger, star anise, and mushrooms, and cook for 1 minute.
Turn the heat back up to high, and add the napa cabbage. Stir-fry for a minute, and then add the salt, oyster sauce, dark soy sauce and chicken stock (or water). Mix everything well, cover, and simmer for 3 minutes. Mix in the cooked glass noodles and pork belly, cover, and simmer for another 2 minutes. Stir in the chopped scallion (if using) and serve!