Heat the mashed potato in the microwave following pack instructions.
Put the cheese sauce and fish sauce (if using) in a large pan and stir over a high heat for a few minutes until bubbling.
Add the fish pie mix together with the prawns and cook for 5 minutes, or until the fish is cooked through. Try not to stir the mixture too much to avoid breaking up the pieces of fish.
Stir in the lemon zest, parsley and some freshly ground black pepper and pour into a deep, ovenproof dish.
Spoon over the mashed potato and cook under a preheated grill for 5 minutes until lightly golden on top. Serve immediately.
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