Take a rectangular pyrex dish or any pie dish you want to use, cut chicken into cubes and add to the dish, taking care not to overfill. Add the sweetcorn, soup and mushrooms if using and stir it all together till all the chicken is coated in soup.
Lay the pastry over the top of pie dish and trim to size, allowing a little overhang as the pastry will shrink some when cooking. Brush with egg or milk and score a few lines across.
Bake in the preheated oven following the cooking times on puff pastry packet, about 25 minutes. Pie will be done when the pastry is golden brown and the filling is bubbling. Remove from the oven and serve.