Sauté the halved ginger and onion in a dry pan until the edges are lightly charred, then set aside; this step gives the stock its characteristic smoky aroma.

Bring the bone-in chicken thigh meat to a boil in cold water in a large pot, skimming off the foam that collects on the surface with a spoon.

Add the charred ginger-onion, cinnamon stick, star anise, salt, and fish sauce, and cook with the lid ajar over low heat until the stock deepens in flavor.

Remove the cooked chicken from the water and shred it, then strain the remaining stock through a fine sieve to remove the spice pieces.

Divide the boiled rice noodles among bowls, pour the shredded chicken and hot stock over them, and serve with spring onion, cilantro, and lemon wedges.