<Sophie Conran's Chicken and Mushroom Pie>

Ingredients

• 1 (1.6kg/31/2lb) whole chicken
• 1 onion, halved
• 1 large carrot, peeled
• 1 stick of celery, sliced
• a few sprigs of parsley
• 25g/1oz of butter
• 45ml/3 tbsp plain flour
• 2250g/8oz button mushrooms, halved
• 45ml/ 3 tbsp chopped fresh parsley
• 1 (300g) pack ready made puff pastry
• 1 egg, beaten
• salt and freshly ground black pepper