Put all of the above ingredients (minus the puff pastry but including your seasoning) in your crock pot and fill with hot water until the ingredients are covered (about 5-6 cups).
Cook for at least 4 hours, but up to 8. If you want to thicken the sauce even more you can add gravy granules, a tsp of bullion, or make a roux.
Roll out your puff pastry and cut in to long rectangle strips and spoon in some mixture into the centre of the bottom half, taking care to keep well within the edges.
Fold the top over the bottom of the pastry and seal the edges with a pinch or twist and poke small holes in the top for steam to come out.
Top the pastries with an egg+water mixture for extra color and browning and cook at 400 degrees for about 15-20 minutes
Top with some extra mushroom-chicken mixture before serving if desired.
Servings 4