Preheat the oven to gas 6, 200°C, fan 180°C. In a large pan, melt the butter, then add the onion and celery and cook until softened. Drain on paper towel and set aside to cool. In a bowl, combine the cooled vegetable mixture with the sausage meat, apple and parsley. Season well.
Cut a third off the pastry block and set aside. Roll out the bigger piece until large enough to cut a 28cm (11in) disc from it. Put the pastry disc on a baking tray lined with nonstick baking paper. Cover the pastry with the sausage meat mixture, leaving a 2·5cm (1in) border. Roll out the remaining piece of pastry until large enough to cut a 20cm (8in) disc from it. Place the disc over the filling.
Brush the base border with beaten egg, then bring it up over the the lid, pinching the edges to seal. Make a couple of small slits in the middle of the pastry lid (this will allow the steam to escape), and brush all over with the beaten egg. Bake in the oven for 25 minutes, until cooked through and golden.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.