In a large stock pot, heat the oil. Season the chicken generously with salt and brown over medium-high heat until the skin is crispy, about 3 minutes per side. Remove to a plate. Add the onion (cut side-down), ginger, and garlic. Cook for two minutes, until the onion is charred. Add the chicken back to the pot along with the cinnamon stick and 4 quarts of water. Bring to a boil, then reduce the heat to medium-low. Simmer until the chicken is fork tender, about 30 minutes, then remove to a plate. Shred the meat and set aside. Return the bones to the pot and continue to simmer until the liquid has reduced by a third, about 1 hour.
Meanwhile, cook the rice noodles according to package instructions. Arrange the garnishes on a plate.
Strain the broth through a fine mesh sieve and return to the pot. Stir in the fish sauce and taste for seasoning, adding more salt as necessary.
Divide the rice noodles and shredded chicken between 4 bowls. Ladle the broth on top and garnish with the herbs, bok choy, chiles, scallions, and lime wedges.