Dice the onion and fry gently in a little oil for two minutes. Slice the mushrooms. Mix meat and veg with the soup. Place half a dozen chestnuts into a plastic bag. Squash with a rolling pin until broken into small pieces. Roll out pastry into an oblong shape. Sprinkle chestnuts over the surface. Fold into three, as shown, and roll again. Place pie filling down the middle of the pastry.
To make the plait:
Make diagonal cuts along both sides of the pastry about an inch apart. Brush with beaten egg. Fold the pastry strips alternately to form the plait. Brush the top with beaten egg. Serve with salad and plenty of alcohol.