Heat oil in a large casserole pot over medium heat. Add the curry paste, garlic and ginger and cook, stirring, for 1 min or until aromatic.
Add capsicum, coconut milk, stock, 2 cups (500ml) water, fish sauce and sugar to pot and cook, stirring, for 5 mins.
Add chicken and cook, stirring, for 3 mins or until chicken is cooked through. Add noodles. Cook, stirring, for 4 mins or until noodles are tender. Add pak choy. Cook for 1-2 mins or until pak choy just wilts.
Ladle soup among bowls. Top with coriander, spring onion and chilli, if using.
