Preheat the oven to 180°C (350°F/ Gas 4). Place the pastry sheets on the counter next to each other and allow to thaw. Wipe the mushrooms clean and tear them roughly into pieces. Fry them while stirring on a very high heat in a little olive oil and season with salt and pepper. Transfer to a plate to cool. Grind the game in a food processor or ask the butcher to do it for you. Combine the mince mixture with the remaining ingredients and knead into a firm ball. Add as much breadcrumbs as necessary, the mixture must remain creamy, not too wet, but not too dry either. You can test it by rolling a small soup ball of the mixture, and heating it for 2 minutes in the microwave to taste whether it is okay. You can decide whether it needs more salt. You have to season mince more than you think, since once fried, it loses a lot of its flavour. Create an oval ball with the mince. Stack the pastry squares.
Dust the countertop with flour. Roll out the stack into a long sheet, 2 times bigger than the meat ball. Place the meat ball on one side of the pastry and fold the other side over it. Brush water on the edge and press firmly in place. If you are a little handy you can decorate the pasty: for example close it along the top with a frill made with your fingers.
Place the pasty on a greased baking sheet and using an apple corer or large fork, make decorative holes in the dough: they will be necessary later to allow the fat to drain. Brush the pasty with lightly beaten egg yolk. Bake the pasty until golden brown, (approx. 45 minutes). Leave to fully cool before slicing. Serve with the cranberry, walnut & pear chutney.