Place onion, garlic, carrot, celery, chilli, ginger, chicken, curry powder, cumin, garam masala and stock in a 4.5-litre (18-cup) slow cooker. Cook, covered, on low, for 8 hours.
2.
Carefully remove chicken from the cooker; discard the skin and bones. Shred meat coarsely using two forks.
3.
Skim fat from the surface of the vegetable mixture. Blend or process vegetable mixture until smooth. Return chicken to the pan with the coconut milk; stir to combine. Ladle soup into serving bowls; sprinkle with coriander to serve, and drizzle with extra coconut milk, if you like.
Garam masala is a blend of spices including cloves, cardamom, cinnamon, coriander, fennel and cumin, that are roasted and ground together. Black pepper and chilli can be added for a hotter version.