Line a baking sheet with baking paper. Unroll the pastry, place a pie dish on it upside down and cut around it. Place the pastry on the prepared sheet, brush with beaten egg and bake in a preheated oven, 200˚C (400˚F), Gas Mark 6, for about 10 minutes, until puffed up and pale golden.
Meanwhile, heat the oil in a large frying pan and cook the leeks for 3–4 minutes, until softened. Add the mushrooms and cook for a further 3–4 minutes, until soft and golden. Add the remaining ingredients and season to taste.
Once the chicken is hot, put the filling into the pie dish, top with the pastry lid and return to the oven for 3–4 minutes, or until the pastry is golden and crisp.