Roast the onion and ginger over an open flame or in a dry pan until the skins are blackened and charred.
In a dry pan over medium heat, toast the coriander seeds, star anise, cinnamon, cloves, and cardamom for 1-2 minutes until fragrant, then place in a spice bag.
Add the chicken, extra bones, charred onion, ginger, salt, and water to a large pot and bring to a gentle boil.
For the first 20 minutes, frequently skim off any foam to ensure the broth remains clear.
Add the prepared spice bag to the broth and allow the chicken to simmer for 30–45 minutes until fully cooked.
Prepare an ice bath and immediately submerge the cooked chicken for 5 minutes to stop the cooking process and firm the skin.
Pat the chicken dry and brush the skin with a mixture of oil and turmeric for a golden finish.
Hand-shred or slice the meat, then return the remaining bones to the pot to simmer for another hour for extra richness.
Strain the broth and adjust the final seasoning with fish sauce, chicken bouillon, and sugar.
Prepare noodles in a separate pot of water according to package directions, then drain.
Assemble by placing noodles in a bowl, topping with chicken and onions, and pouring the boiling hot broth over them.
Serve immediately with fresh herbs, bean sprouts, lime, and dipping sauces.