Step 1: Char the unpeeled ginger and shallots over an open flame or under the broiler until blackened. Rinse off the burnt skin and lightly smash the ginger.
Step 2: Toast the star anise, cinnamon, cloves, and coriander seeds in a dry pan over medium heat for 2-3 minutes until very fragrant.
Step 3: Place the whole chicken in a large pot, cover with water, and boil for 5 minutes. Drain and rinse the chicken and pot to ensure a clear broth.
Step 4: Refill the pot with 4 quarts of fresh water. Add the chicken, charred aromatics, toasted spices, and rock sugar. Simmer gently for 45-60 minutes, skimming any foam that rises to the surface.
Step 5: Remove the chicken and place in an ice bath. Once cool, shred the meat and discard the bones (or save for future stock).
Step 6: Strain the broth through a fine-mesh sieve. Stir in the fish sauce. Keep the broth at a very low simmer until ready to serve.
Step 7: Cook the rice noodles according to package instructions. Divide noodles into 4 large bowls.
Step 8: Top the noodles with shredded chicken and sliced onions. Pour the boiling hot broth over the chicken and noodles. Serve immediately with a side plate of fresh herbs, sprouts, and lime.