Heat the oil in a saucepan, add the chicken and curry paste and fry for 3–4 minutes until just beginning to colour. Stir in the galangal paste, lime leaves, coconut milk, fish sauce and muscovado sugar, then mix in the stock.
Bring to the boil, cover and simmer for 15 minutes, stirring occasionally until the chicken is cooked.
Create curls by cutting very thin strips from the two spring onions reserved for a garnish. Soak in cold water for 10 minutes, then drain.
Add the remaining spring onions, mangetout and bean sprouts and cook for 2 minutes. Ladle into bowls and tear the coriander leaves over the top. Scatter the spring onion curls over the soup.