Put a baking tray in the oven and preheat the oven to 200 degrees C.
Melt the butter in a large frying pan over a moderately low heat. Add the mushrooms and gently cook, stirring occasionally for 4–5 minutes until tender.
Put the mascarpone or cream cheese in a large bowl and lightly beat until softened.
Reserve 1 1/2 tablespoons of the milk then pour the rest into the bowl with the mascarpone or cream cheese along with the mustard. Stir together to make a thick sauce.
Add the chicken, chives and the mushrooms with their juices to the creamy sauce. Season to taste with salt and freshly ground black pepper, then spoon the mixture into a 1 litre pie dish. Place a pie funnel or an upturned ovenproof china eggcup, if you have one, in the middle of the filling to support the pastry lid.
If necessary, roll out the pastry on a lightly floured surface so that it is about 5 cm larger all round than the pie dish. Cut off a 3 cm strip around the edge of the pastry sheet, then press this strip onto the rim of the pie dish. Lightly brush with water, then lay the pastry lid on top, pressing the edges together to seal.
Trim the excess pastry, pinch to seal, then ‘flake’ the edges. To do this, hold a sharp knife horizontally to the side of the pastry edge and make shallow cuts all the way around. Re-roll the pastry trimmings and cut out leaf shapes to decorate and stick them onto the pastry lid with a little of the reserved milk. Glaze all over with the remaining milk and cut two slits in the top to allow steam to escape.
Place the pie dish on the hot baking tray and bake for 25–30 minutes, or until the pastry is nicely puffed and golden brown. Leave to stand for 5 minutes before serving, sprinkled with pepper.