Preheat the oven to 200C/400F/Gas 6.
Sauté the shallot in a little olive oil. After a few minutes add the fish stock and cream and bring to the boil. Season and leave to one side.
Roll out the pastry to twice the size of the ovenproof dish.
Place the fish, mushrooms and chives in the dish. Season and cover with the sauce.
Spread half of the pastry with the mustard and cheese. Fold the other half over the top, sandwiching the mustard and cheese in the middle.
Top the pie with the pastry, crimp the edges and then egg wash.
Bake for 25-30 minutes.