Preheat your oven to 200°C/180°C fan. Add the halved potatoes to a saucepan of salted water and bring to the boil. Cook for 15 minutes, until tender, a knife inserted into a potato should slide in easily. Drain, then return the potatoes to the pan and leave to steam dry for a couple of minutes.

Whilst the potatoes are cooking, set a large frying pan over medium heat and add half the butter. Once foaming, add the sliced leeks and cook for 10 minutes, until tender. Remove the pan from the heat and add the peas, crème fraiche and chopped parsley leaves. Season to taste with salt and black pepper. Stir in the fish and cooked potatoes and pour into a deep-side baking dish.

Melt the remaining butter. Take a sheet of filo pastry and brush one side with melted butter. Cut the pastry in half widthways and scrunch up each half like a tissue, this will give your pie a deliciously crunchy top. Sit the pastry on top of the pie fish mixture and repeat with the remaining pastry.

Bake for 30 minutes, until golden and bubbling. Check after 15 minutes and cover with foil if it's browning too quickly. Leave to stand for 10 minutes before serving (if you can wait!).