Make the sauce: Combine the water, golden mountain seasoning sauce, white sugar, dark soy sauce, fish sauce, white vinegar, and chili flakes in a small saucepan over medium heat. Stir until the sugar dissolves and the sauce begins to bubble, then remove from the heat.
Cook the pork: Bring a pot of water to a boil. Add the thinly sliced pork and cook until fully cooked through. Remove and drain well.
Prepare the noodles and vegetables: Cook the rice noodles according to the package instructions, then drain thoroughly. Blanch the chopped Chinese broccoli in boiling water for about 10 seconds and drain.
Toss everything together: Add the cooked pork, noodles, and Chinese broccoli to a large mixing bowl. Pour over enough sauce to coat everything and toss well, adding more sauce as needed.
Garnish and serve: Top with fresh bean sprouts, chopped green onions, and coriander. Serve immediately with extra chili flakes if desired.