Photograph: Clare Winfield
Rub the insides of a 1.5-litre pie dish (with a lip) with a knob of the butter (about 10g); set aside.
Melt 50g of the remaining butter in a frying pan over a low heat. Add the leeks and a pinch of salt and fry gently, stirring occasionally, for 5-10 minutes until softened but not coloured. Tip the leeks into the bottom of the pie dish.
Mix the stock and the milk together in a pan and bring to the boil. In a separate pan, melt the remaining 40g butter. Add the flour and cook, stirring, for 2-3 minutes. Slowly add the hot stock and milk, whisking until thick. Add the lemon zest and nutmeg. Season; leave to cool, then stir in the parsley and tarragon.
Scatter the fish over the leeks in the pie dish; top with the prawns. Pour the cooled sauce over the top. Chill for 1 hour.
Roll out the puff pastry on a lightly floured surface to a 28 x 20cm rectangle that's 5mm thick. Cut it into strips the same length as the pie dish, each about 1.5cm wide. On a large lightly greased baking sheet, arrange evenly spaced strips of puff pastry in one direction. Next, interweave more strips in the opposite direction, going under and over to create a lattice. Chill for 1 hour until firm.
When you're ready to bake, preheat the oven to 200°C, fan 180°C, gas 6, and place the pie on a baking sheet. Beat the egg yolk with the 2 tbsp of milk and brush the rim of the pie dish with the mixture. In one swift motion, slide the lattice on to the pie; press to seal. Brush the pastry strips with the remaining beaten egg mixture. Bake for 45-55 minutes until the pastry is golden. Serve with peas or salad.
Get ahead: make up to the end of step 5 up to 6 hours ahead