Preheat the oven to 400 degrees F. Line a large baking sheet with parchment paper and set aside.
Remove the pie dough from the fridge and unwrap the dough. Sprinkle some flour onto a clean work surface and use a rolling pin to roll out the dough. Make sure to keep the dough in a large round shape, aiming for a thickness of 6-8mm.
Roll the dough up onto the rolling pin and transfer to the parchment-lined baking sheet. Unroll the dough so that it is in the center of the baking sheet.
Place the ricotta cheese in the center of the dough and spread it out into a circle. Make sure to leave at least a 2-inch border around all the sides.
Top the ricotta cheese with the mushrooms, piling them higher in the very center of the galette.
Next, fold the edges of the dough towards the center, making sure to leave some of the mushroom filling exposed in the middle. Repeat this all the way around the galette.
Combine the egg and milk in a small bowl and use a pastry brush to coat the dough on top of the galette.
Bake the galette for 40 minutes in the center of the oven, turning halfway to ensure even browning. Once cooked, removed from the oven and let cool on the pan for at least 10 minutes. Serve warm and enjoy!