Put the potatoes in a steamer and steam for about 12 minutes, until they are tender.
Alternatively, cook the potatoes in boiling water, but make sure you drain them well.
Melt the butter in a large saucepan and, when bubbling, add the spring onion and bay leaf. Cook for about 2 minutes, until softened.
Stir in the flour for about 30 seconds before gradually adding the fish stock, stirring to prevent any floury lumps forming.
Bring to a simmer and cook for a few minutes, until you have a thick sauce.
Stir in the potatoes, crayfish, sweetcorn, herbs and cream.
Season and bring to a simmer before spooning into a shallow pie dish.
Roll the pastry out on a lightly floured surface, so it fits your pie dish with some overhang.
Brush the edges of the dish with the beaten egg and lay the pastry on top.
Press the edges of the pastry with the back or handle of a fork to crimp and seal all around.
Trim any overhanging pastry. Brush with beaten egg and use a sharp knife to pierce a hole in the top to allow steam to escape while cooking.
You can use any leftover pastry trimmings to
decorate the top of the pie, or just leave it plain.
Place the pie on a baking tray and bake for 20–25 minutes, until the top of the pie is golden and the filling is piping hot.