Pulse flour, baking powder, salt, butter, and egg yolk in a processor until resembling breadcrumbs.
Add water gradually until dough forms, then wrap and chill for one hour.
Brown the game and venison meat in batches using vegetable oil and set aside.
Sauté bacon and onions for 4 minutes, then add celery and mushrooms until tender.
Stir in flour for 2 minutes, then add bay leaves, jelly, stock, wine, and thyme.
Simmer the mixture for two hours until meat is tender and then allow to cool.
Preheat oven to 180°C and grease a 20cm deep pie tin with butter.
Roll out shortcrust pastry and line the prepared pie tin.
Blind bake with parchment and beans for 20 minutes before removing beans to cool.
Fill the baked pastry case with the cooled meat mixture.
Top with a sheet of puff pastry, sealing the edges with beaten egg.
Crimp edges, trim excess pastry, and prick the top for steam ventilation.
Glaze with remaining egg and bake for 30 minutes until golden-brown and risen.
Cool the pie completely before serving in thick slices.