Preheat the oven to 180°C fanCut off ¼ of pastry dough, and retain for lid. Roll out remaining dough into a circle at 0.5cm thickness.Line a 20cm non-stick springform cake tin, pressing the pastry against the sides and base, leaving a 2cm overlap on the top of the tin to join the lid. Chill this pie case in the fridge for 10 minutes, and then pack in the meat farce.Roll out the dough for the lid as before, and place on the top. Crimp the pastry over together where it meets to make a seam, and then make a small hole in the centre of the lid for steam to escape. Brush the top of the pie with the beaten egg yolk.Cook in the oven for 1 hour, or until internal temperature reaches 65°C in the middle then leave to cool overnight.Soak gelatine in cold water for 5 minutes.Then boil the cider in a pan until reduced by half. Add the chicken stock and sage stalks, and turn off heat. Squeeze excess water off the gelatine, and whisk into stock.When fully incorporated, pass through a sieve and carefully pour into top of pie through a small funnel.Allow to set in fridge for 1 hour then remove from the cake tin, ready to serve.