Heat the oil in a large heavy-based saucepan then add the curry paste. Cook, stirring, for about 30 seconds. Pour in the stock and the coconut milk, then stir in the sugar and let it simmer for 5 minutes.
Add the chicken fillets and cook for a further 5 minutes. Toss in the mushrooms and cook for 3 minutes, then add the sugar snaps. Simmer for a few minutes more, until the chicken is cooked through and the vegetables are just tender.
Squeeze in the lime juice, then ladle the soup into bowls and top with spring onions, beansprouts, coriander leaves and a little lime zest.
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