Step 1: Char the aromatics by placing the halved onion and sliced ginger directly over a gas flame or under a broiler for 5-7 minutes until blackened in spots.
Step 2: Parboil the whole chicken in a large pot of water for 10 minutes. Drain and rinse the chicken thoroughly under cold water to remove impurities.
Step 3: In a small dry pan, toast the star anise, cinnamon, cloves, and coriander seeds over medium heat for 2-3 minutes until fragrant.
Step 4: Place the rinsed chicken back into the clean pot. Add 4 quarts of water, the charred onion, charred ginger, toasted spices, fish sauce, and sugar.
Step 5: Bring to a gentle simmer. Cook for 1 hour, skimming any foam that rises to the top. Do not let it boil vigorously.
Step 6: Remove the chicken from the pot and let it cool slightly. Shred the meat and discard the skin and bones (or return bones to the pot for extra flavor).
Step 7: Strain the broth through a fine-mesh sieve into a clean pot. Taste and adjust seasoning with more fish sauce or sugar if needed.
Step 8: Prepare the rice noodles according to package directions, then drain.
Step 9: Assemble the bowls by placing a portion of noodles in each, topping with shredded chicken, and ladling the hot broth over them.
Step 10: Serve immediately with a side plate of fresh herbs, lime, and chilies for customization.