Roast Aromatics: Roast onion and ginger under a broiler or over an open flame for 5–7 minutes until slightly charred.
Make Broth: In a large pot, place chicken and cover with broth or water. Add roasted onion, ginger, garlic, star anise, and cinnamon. Bring to a boil, then simmer 45–60 minutes. Skim foam occasionally. Season with fish sauce, salt, and pepper.
Prepare Noodles: Soak or cook rice noodles according to package instructions. Drain and set aside.
Shred Chicken: Remove chicken from broth, let cool slightly, then shred meat into bite-sized pieces.
Assemble Soup: Divide noodles into bowls. Add shredded chicken, bean sprouts, and fresh herbs. Pour hot broth over the top. Garnish with lime wedges, chili slices, and optional sauces.
Serve: Enjoy hot immediately, adding more herbs or sauces to taste.