1) Rub 175g of the butter and 350g of the plain flour together until mixture resembles fine breadcrumbs. 2) Add sufficient cold water to form a firm dough. Cover and chill for 30 minutes. 3) Meanwhile, heat oil and fry sausages for 10 minutes. Remove from pan, remove skins, chop and leave to cool. 4) Melt the remaining 25g of butter in pan, add onions, bacon and celery and sauté for 5-6 minutes. 5) Stir in remaining 25g of flour and cook for 1 minute. 6) Remove from heat and gradually add milk. 7) Return to heat and stir until sauce thickens and boils. 8) Add cheese, sausages, parsley and seasoning. Stir well, cover and leave to cool. 9) Preheat oven to 200°C, Gas 6. 10) Grease a 24cm flan tin. Roll out half of the pastry and line tin. 11) Spread the filling evenly over the pastry base. 12) Roll out remaining pastry and cover pie. 13) Decorate with pastry trimmings and make 3 vent holes in top. 14) Brush with beaten egg and bake for 30-35 minutes, until golden.