Broth Preparation: In a large pot, combine chicken thighs, water, lemongrass, ginger, and garlic. Heat until it reaches a boil, then reduce to a simmer and continue cooking for 45 minutes.
Noodle Making: Mix rice flour, tapioca starch, 1 cup warm water, and oil in a bowl to form a dough. Roll into small balls and flatten into noodles.
Cook Noodles: Bring another pot of water to boil, cook noodles until they float, about 3-4 minutes, then drain.
Assemble the Soup: Remove chicken from the broth, shred it, and return to the pot. Season broth with fish sauce, salt, and pepper.
Serving: Place cooked noodles in bowls, ladle the hot broth and chicken over them, and garnish with scallions, fried garlic, lime wedges, and cilantro.