Prepare the mushrooms and simmer them for 10 min in milk seasoned with salt and pepper.
Chop the ham roughly and mix with the peas, hard-boiled eggs, mushrooms, mustard and seasoning.
Divide the pastry into 2 unequal portions.
Roll out the larger piece and cover a fireproof dish or plate and trim the edges.
Put in the prepared filling.
Roll out the remaining pastry and cover the pie.
Flake and scallop the edges and decorate as desired.
Glaze top with beaten egg or milk.
Bake in a very hot oven (450° F., Gas 8) until set and then reduce heat to warm (335° F., Gas 3) for the remainder of the time.
Cook for about 45 min in all.