Preheat the oven to 200oC, gas mark 6.

Heat 1 tbsp oil in a large frying pan and add the squash, leeks and thyme and fry for 5-6 minutes. Stir in the mushrooms and redcurrant jelly and fry for a further 5 minutes. Stir in the 100g walnuts and breadcrumbs, season well and allow to cool.

Roll out the pastry on a lightly floured surface to a 30x35cm rectangle. Place the stuffing in a line in the middle along the length of the pastry leaving a 4cm border at the side edges. Fold the pastry at the sides over the filling then fold over the rest of the pastry to enclose the filling.

Transfer to a baking tray, seal side down. Brush the surface with egg. Sprinkle over the reserved 1 tbsp chopped walnuts. Bake for 30 minutes until golden.