Soak glass noodles in plenty of water. Thinly slice pork and mix with 1/4 teaspoon salt, 1/8 teaspoon black pepper and 1 teaspoon cooking oil. Set aside. Prepare all other ingredients.
Heat a wok (or a thick and heavy deep skillet) until it is very hot. Add some oil to lightly coat the bottom, then sear pork in a single layer over medium-high heat until both sides are browned (about a minute each side). Transfer pork to a plate and set aside.
Add mushrooms to the wok and also sear until both sides are browned. This may take 1½-2 minutes each side. Transfer mushrooms to a plate and set aside.
Lower the heat to medium. If there is a lot of oil in the wok, remove most of it and just leave enough to lightly coat the bottom. Add minced garlic and sauté for several seconds until fragrant. Then add oyster sauce and stir for a few seconds. Add about ⅓ cup (80 ml) water and wait for it to start boiling.
Add onion and carrot to the wok and let it come back to a simmer. Dissolve cornstarch with ½ tablespoon of water and slowly pour the cornstarch slurry into the pan while stirring.
Place seared pork slices over the onion and carrot, followed by the mushrooms. Then place half of the napa cabbage and half of the glass noodles over the mushrooms. Top with the remaining cabbage and noodles, alternating them, with some cabbage at the very top layer.
Pour the remaining water over the ingredients in the wok. Cover and braise over medium heat for about 20 minutes. Adjust the heat and cooking time as needed. You can check the doneness of cabbage and noodles after 15-18 minutes and add more time as needed.
Open the lid and there may be a bit of liquid left in the wok. Use a pair of tongs to gently toss/mix the noodles with the pork and cabbage. They will soak up any remaining sauce and absorb the flavors. Also taste and adjust seasonings as needed. Remove from the heat.
Divide the dish into serving bowls and serve right away.