Garnish: Prepare garnish ingredients, place in individual serving dishes and set aside. Gravy: Wash sweet potatoes, pierce several times with a sharp knife, and steam (or cook in microwave) until tender. Set aside to cool. Mince onion and chiles, mix together and grind into a coarse paste (or continue chopping until this consistency is achieved). Peel and chop sweet potatoes. In a medium bowl, mash 3/4 of the potatoes, adding vegetable stock (up to 1 cup) as needed. Heat large skillet or wok over med-high heat. Toast coriander in heated pan until very fragrant, 2 to 3 minutes. Remove coriander from the pan. Dry-fry onion and chile paste, stirring frequently, for 3 minutes. Add several Tbsp vegetable stock, preserved bean curd, and toasted coriander to the pan. Mash the bean curd, mixing with other ingredients in the pan and fry for 2 minutes. Add remaining stock. When the stock comes to the boil, blend in mashed potatoes, reduce heat to low, add reserved potato cubes, cover, and simmer. Noodles and Bean Sprouts: Bring a large pot of water to the boil. Blanch bean sprouts (2 or 3 minutes) and divide among 4 wide soup bowls. Cook noodles according to type used, and divide among bowls. Stir gravy, adding warm stock or water if needed to thin, and quickly ladle over hot sprouts and noodles. Serve with garnishes to add as desired.