Put the rice in a pan with 500ml cold water. Bring to the boil, then turn down to a simmer and cook, covered, for 14 minutes.
Meanwhile, heat the oil in a large pan. Add the curry paste, coriander stalks, green beans and most of the chilli and cook over a low heat for 5 minutes, then pour in the fish stock. Bring to a gentle simmer, then add the fish sauce. Cook for a further 5 minutes.
Stir in the fish pie mix and cook for 1 minute. Add the spinach and coconut milk, then cook gently for 2 minutes, but don't boil. Stir through the lemon juice and most of the coriander leaves. Garnish with the remaining coriander and chilli and serve with the rice.
Cook's tip: for a thicker curry, use full-fat coconut milk.