Put the smoked haddock into a shallow pan and pour the milk over. Bring to a simmer and poach for 3-5 minutes until just cooked. Remove the fish, set aside and reserve the poaching milk.
In another pan, melt 50g of the butter, stir in the flour and cook for 1 minute. Remove from the heat, then mix in the poaching milk, a little at a time, beating with a wooden spoon after each addition. Bring to the boil, stirring. Season, lower the heat and cook for 2 minutes. Add the cream and parsley. Simmer for 1 minute.
Put the smoked haddock and cooked prawns into a 2-litre pie dish. Stir any milky liquid under the fish into the sauce, then pour all the sauce over the fish and mix through.
Meanwhile, melt the remaining 50g butter in a saucepan and add the leeks. Turn until coated in butter. Season, add a couple of tablespoons of water, cover with a lid and sweat over a low heat until really soft.
Preheat the oven to 200°C, gas mark 6. Cook the potatoes until tender, then drain thoroughly and mash, adding the leeks and buttery juices. Season well. Top the pie with the mash. Bake for 30 minutes until golden and piping hot.
This recipe was first published in Sun Nov 01 00:00:00 GMT 2009.