Heat the oven to 180C. Prick the sweet potatoes with a fork and cook until soft (this can take up to 2 hours depending on the size of your sweet potatoes, so plan/make ahead!). Baking them like this is absolutely worth the effort! Remove the skin and mash the flesh with 2 tbsp of the oil, the cumin and paprika. I like to use a stick blender to get it beautifully smooth, but this is not essential. Set aside and keep warm.
Meanwhile, heat a frying pan with a glug of oil and cook the onions and peppers until soft.
In a separate saucepan, warm the milk over a low-medium heat. Then add the fish pie mix and prawns and cook for 4 mins until the prawns have just turned pink. Remove the the fish and prawns and reserve the milk.
Add the harissa paste to the onions and peppers and cook for 2 mins.
Add the reserved milk bit and cream to the onions and peppers. When the mixture starts bubbling, combine the cornflour with a little milk and add - stirring the whole time. Once it starts to thicken, add the spinach and cook until wilted. Lastly, add the fish and prawns, stirring carefully.
Pour the fish mixture into an ovenproof dish, then spoon over the sweet potato mash. Carefully spread it out, then rough it up with a fork.
Bake for 20 mins, then grill for 10 mins on the hottest setting. Serve with cooked greens of your choice.