Hot Smoked Fish & Leek Pie

Serves 8
Leeks 500g
Butter 40g
Plain flour 3 heaped tbsp
Hot smoked salmon 250g
Smoked mackerel 250g
Double cream 500ml
New potatoes 300g
Olive oil 4 tbsp
Chopped parsley 3 tbsp
Chopped tarragon 2 tbsp
Puff pastry 375g
Egg 1
Nigella or sesame seeds 1 tbsp

You will need two large baking sheets and a piece of baking parchment.

Cut the leeks into discs 1cm in width, wash thoroughly, then put them in a deep heavy-based pan with the butter over a moderate heat. Let the leeks cook for eight to 10 minutes, covered with a round piece of greaseproof paper and a lid, so they cook in their own steam and soften without browning.
Gently warm the double cream in a small pan and remove from the heat. Add the flour to the leeks, stir and leave to cook for a couple of minutes then break the salmon and mackerel into large flakes and gently stir into the leeks. Pour over the warm double cream and leave to cook over a low heat for 5 minutes. Remove from the heat.
Slice the potatoes into thin coins, each about the thickness of a £1 piece. Warm the olive oil in a shallow pan then fry the potato slices on both sides over a moderate heat till golden. They should be soft inside. Fold the cooked potatoes into the fish and leek cream, season carefully with salt and black pepper, the chopped parsley and tarragon, transfer to a mixing bowl and leave to cool. Chill thoroughly in the fridge. (If you skip this step it is impossible to shape the pie.)
Set the oven at 200c/gas mark 6. Place one of the baking sheets upside down in the oven. Line the other one with baking parchment. Cut the pastry in half then roll out each piece to a rectangle to a rectangle 32cm x 22cm. Place one rectangle on the lined baking sheet. Pile the cold filling on top of the pastry, leaving a border on all four sides of at least 2cm. Smooth the top of the filling so you have a deep rectangle of mixture. Beat the egg in a small bowl or cup and brush the bare edges of the pastry generously.
Place the second sheet of pastry over the filling then press the edges of the two pieces of pastry firmly together to seal. It is important that they are well sealed, otherwise your filling will leak. Brush the top layer of pastry all over with the beaten egg, scatter with the nigella or sesame seeds then pierce a small hole in the centre with a knife or the handle of a wooden spoon. Place in the oven, the baking sheet on top of the hot, upturned one, and bake for 40 minutes till golden brown.
Leave to settle for 5 minutes before sliding off the baking sheet onto a serving board or dish, and slicing.