Preheat the oven to 200°C (gas 6) and place a baking sheet inside to heat. Melt the butter in a large frying pan and gently cook the mushrooms for 4–5 minutes until tender.
Beat the mascarpone or cream cheese in a bowl until softened. Reserve 2 tbsp of the milk, then mix the rest into the cheese with the mustard to make a thick sauce. Add the chicken, chives and the mushrooms with their juices to the cheese mixture. Season to taste, 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.
Roll out the pastry on a lightly floured surface so that it is about 5cm larger all round than the pie dish. Cut off a 3cm strip around the edge, then press this strip on to the rim of the pie dish. Lightly brush with water, then lift the pastry lid on top, pressing the edges together to seal.
Trim the excess pastry, pinch to seal, then ‘flake’ the edges (see below). Re-roll the pastry trimmings and cut out leaves 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 on the hot baking sheet 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.
This pie freezes well - simply assemble the pie and freeze before baking. Once you're ready to bake, simply throw it in the oven straight from the freezer.
*For a chicken and leek pie, leave out the mushrooms and cook two sliced leeks in the butter until soft. *For cooked chicken, a hot roasted chicken from the supermarket is likely to be cheaper, and more convenient, than buying chicken portions. *To ‘flake’ the edges, hold a sharp knife horizontally to the side of the pastry edge and make shallow cuts all the way round.