Preheat the oven to 180°C.
Mix minced pork, game, eggs, thyme, chestnuts, and allspice in a bowl and season.
Roll the puff pastry on a floured surface to a 30x20cm rectangle.
Shape the meat mixture into a log, place on the pastry, roll to enclose, and seal the edges with a fork.
Brush with egg yolk and bake for 15-20 minutes until golden-brown.
Heat butter in a pan and fry mushrooms for 5 minutes until the moisture evaporates.
Add the double cream to the mushrooms and bring to a boil.
Blend the mushroom mixture into a coarse purée and pulse in the mustard.
Slice the sausage roll and serve with the mushroom garnish, pickled walnuts, and dandelion leaves.