Preheat the oven to 200C/gas 6. Boil the potatoes for 15-18 minutes, or until tender. Drain well, return to the pan and add the herbs and fromage frais. Season well, mash and set aside.
Meanwhile, spray a large frying pan with Frylight and place over a medium heat. Stir fry the leek for 4-5 minutes, or until soft. Add the flour, stir well and cook for 1-2 minutes.
Remove the pan from the heat and gradually stir in the stock. Return the pan to the heat and cook, stirring until the sauce has thickened. Simmer gently for about 10 minutes.
Stir in the prawns and peas. Season then pour half the sauce into a medium-size pie dish. Put the haddock fillets on top and spoon over the remaining sauce. Top with the mash, sprinkle with freshly ground black pepper, brush with the beaten egg and bake for 35-40 minutes or until the topping is golden and the fish is cooked through.
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