In a saucepan, melt a little butter and sweat the onion and garlic.
Add the mushrooms and cook until the water has completely evaporated.
Moisten with white wine and reduce to dryness.
Add the ground pork and veal, then the spices.
Cook for 5 minutes, stirring to separate the meat clumps.
Add the grated potato and cook for an additional 10 minutes.
Season and let cool.
Add the shredded braised pork shoulder to the previous preparation and mix well.
Roll out the puff pastry on a floured work surface and line a pie plate.
Fill with the meat preparation and cover with a circle of puff pastry.
Brush the tourtiere with the egg yolk-milk mixture.
Pierce the top pastry with a knife so that the steam can escape during cooking.
Preheat the oven to 425 F (220 C).
Place the tourtiere in the oven, on the bottom rack, for 25 minutes if you want to freeze it (pre-cook), or 35-40 minutes in total if you want to eat it right away.
To reheat a frozen tourtiere, cook for 45 minutes at 350 F (180 C), covering with aluminum foil if necessary.