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Tip the stock into a saucepan, then stir in the curry paste, fish sauce, sugar, lime juice and most of the zest. Bring to the boil, then add the mushrooms and whites of the spring onion. Cover, then simmer for 2 mins.
Stir in the chicken and most of the spring onion greens to gently heat through, then serve ladled into bowls and scattered with the remaining lime zest. Serve with extra lime juice, sugar and fish sauce on the side so everyone can adjust their own bowlfuls.