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Heat the oven to 180C/160C fan/gas 4. Prick the sweet potatoes with a fork and cook for 50 mins-1 hr or until the flesh is soft (or in the microwave on high for 5-7 mins). Remove the skin and mash the flesh with 2 tbsp of the oil, the cumin and paprika. Set aside and keep warm.
Meanwhile, heat a frying pan with the remaining 1 tbsp oil, and cook the onions and peppers over a medium heat until soft, about 7 mins. Warm the milk in a pan over a low-medium heat and add the fish pie mix and the prawns. Cook for 4 mins until the prawns have just turned pink, then scoop out the fish and prawns. Reserve the milk.
Add the harissa paste to the onions and peppers and cook for 2 mins. Stir through the cornflour, then add the reserved milk bit by bit, stirring as you go, followed by the cream. Add the spinach and cook for 2 mins until it softens, then add the fish and prawns, stirring carefully to avoid breaking them up.
Pour the fish mixture into an ovenproof dish, approximately 25 x 18cm. Carefully spoon the sweet potato on top, going from end- to-end and meeting in the middle. Carefully spread out with a knife, then go over it with a fork to make ridges. Bake for 20 mins, then grill for 10 mins on the hottest setting. Serve with the green beans.