1. Char the aromatics: In a large skillet over medium-high heat, heat the oil then place the onion and ginger cut side down. Cook until blackened and charred in spots, 3-5 mins, then flip and char the other cut side as well. Remove from the pan and set aside.
2. Add the star anise, cinnamon, cloves, coriander seeds, and fennel seeds to the skillet. Toast 1-2 mins until fragrant.
3. Place the whole chicken in a large pot. Add the charred aromatics and toasted spices, then pour in the water, salt, sugar, and fish sauce. Bring to a gentle boil, then immediately reduce to a low simmer. Skim off any gray foam that rises to the surface with a mesh skimmer. (If you accidentally scoop up some of the spices while skimming, just rinse them off and add them back to the pot.)
4. Partially cover the pot with a lid (so steam can still come out), and simmer for 1 hour 15 mins, turning the chicken halfway through. Skim off any additional foam that rises to the surface during cooking to keep the broth clear.
5. Lift the chicken out and transfer to a large bowl or sheet pan, let cool slightly, then shred the meat with your fingers. Discard skin and bones.
6. Strain the broth through a fine-mesh sieve. Taste and adjust seasoning if desired.
7. Prepare rice noodles according to package directions. Divide noodles into bowls, top with shredded chicken, red onion, scallion, and chili. Ladle hot broth over, then serve with bean sprouts, cilantro, and lime wedges.
