Preheat oven to 200°C/400°F/gas mark 6.
Lightly sauté onion and celery in the melted butter until just soft, drain on paper towel and allow to cool. Once cooled mix together the sausage meat with the vegetables, apple and chopped parsley.
Cut one third off the pastry block and roll out the larger piece big enough to cut out a 28cm disc. Lay the pastry on a lined baking tin and spread the sausage meat in an even layer on top, leaving a good 2.5cm border all round.
Roll out remaining pastry large enough to cut out a 20cm disc, place on top of filling.
Brush base pastry borders with beaten egg, then bring up all round and pinch or crimp onto the top piece of pastry around sides to form the pie ‘walls’.
Make a couple of slits in centre of pastry lid (or prick 2-3 times in centre with a fork) and brush all over with beaten egg.
Bake for 25 minutes until golden brown all over.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.