Heat the oil in a large, deep saucepan over a medium-high heat and cook the shallots, lemongrass, garlic, galangal, chillies, coriander stalks and curry paste for 1-2 mins, stirring often until the mixture is fragrant. Pour in the coconut milk and stock, and stir well to release any browned bits on the bottom of the pan. Simmer for 10-15 mins.

Mix in the chicken pieces and lime leaves. Bring to a gentle bubble and cook for 10 mins more until the chicken is cooked through, adding the mushrooms for the final 5 mins. Stir in the fish sauce and sugar, then add lime juice to taste. Add a splash more fish sauce or sugar, if needed. Remove the lemongrass and galangal slices, then serve with the coriander leaves sprinkled over.