Preheat the oven to 200°C, fan 180°C, gas 6. Divide the sliced potato between 4 individual 300ml ovenproof dishes.
Melt the butter in a medium saucepan and add the flour, stirring all the time with a wooden spoon. When you have a thick paste, slowly add the milk, a little at a time, stirring continuously over a gentle heat, until you have a smooth, thick and creamy sauce. Remove from the heat and beat in the crème fraîche, parsley and lemon zest. Gently stir through the salmon and peas and set aside.
Unroll the puff pastry on a very lightly floured surface and, using one of your individual dishes as a template, cut out circles from the pastry to fit the top of the dishes. From the remaining pastry, cut 4 narrow 2 x 20cm strips.
Divide the filling between the 4 dishes. Brush the strips of pastry with the beaten egg and place the strips around the inside top of the dishes just slightly overlapping the edge. Sit the pastry rounds on top of the strips and press them gently together to seal. Crimp the edges of the pies using your fingers or a fork. Brush the lids with the beaten egg and decorate with pastry fish shapes cut from the leftover pastry.
Bake the pies for 25 mins until the tops are puffed and golden.