Unroll the pastry and use a 15 cm (6 inch) saucer as a template to cut 4 circles. Place on a large baking sheet lined with baking parchment. (If you have time, cut leaves from the trimmings and place 4 on top of each circle.) Glaze with beaten egg and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes or until golden and crisp.
Meanwhile, heat the butter in a large, heavy-based frying pan and cook the chicken over a high heat for 10 minutes or until golden. Add the leeks and cook, stirring occasionally, for a further 5 minutes or until soft. Add the parsley and the wine. Stir, increase the heat and boil to reduce the wine by half.
Add the cream and mustard and heat through, without boiling, for 1 minute until piping hot. Season generously and ladle the chicken mixture on to 4 warmed serving plates. Top each with a pastry circle and serve with vegetables.