Char onions & ginger: Heat 1 tbsp oil in a large pot over high heat. Place ginger and onion halves facedown, leave undisturbed for 2 minutes until they blacken. Turn and leave for another 2 minutes.
Add broth ingredients: Put charred onions and ginger into the pot with 2 litres of water, 1.5 kg chicken thighs, coriander bunch, star anise, cinnamon stick, cloves, and fennel seeds. Bring to a simmer.
Gentle simmer: Lower heat to very gentle simmer with lid on, cracked open. Skim off any scum that rises to the surface once or twice during simmering.
Simmer for 1.5 hours. The broth should develop a rich aroma and flavor.
Strain broth: Remove chicken and coriander. Strain broth into a clean pot. You should have about 1.5 liters.
Season broth: Add salt to taste. The broth should be slightly saltier than you prefer.
Shred chicken: Remove chicken meat from bones, discard bones and skin. Shred the meat.
Serve: Keep broth warm. Ladle into bowls over noodles, add shredded chicken, and garnish with fresh herbs.