Preheat oven to 400 degrees F and line a baking sheet with parchment paper.
Melt 1 tablespoon butter over a medium heat in a saute pan.
Add the ½ cup onion and cook onions over a medium-low heat until they begin to sweat. About 10 minutes. Add 3 cloves of garlic. Cook for 1 minute.
Add 1 pound of mushrooms and ½ teaspoon sea salt, cook mushrooms over a medium heat until they have fully browned. About 10 minutes. Then set aside.
Add remaining 1 tablespoon of butter to the pan and melt over medium heat. Add 1 pound butternut squash and saute for 5 minutes.
Add all previous ingredients to a mixing bowl and combine with 1 cup parmesan cheese and 2 tablespoon parsley. Mix ingredients until combined.
Flour your work surface for rolling out dough.
Place the 8-ounce puff pastry sheet on the floured surface and roll the dough out to at least 12-inches by 10-inches.
Spoon the mushroom and butter mixture onto the center of the dough, creating a loaf with the mixture that is parallel to the longer 12-inch side of the dough. Take one 10-inch side and lift it up and over the mushroom butternut mixture, pressing it down around the sides to the shape of the loaf. Repeat on the other side.
Beat 1 egg with ½ teaspoon of water.
Brush the 2 ends of dough covering the mixture with the egg, fully coating the part in contact with the mixture and the sides. Now fold the 2 sides of remaining dough over on top of each other to seal in the mushroom butternut mix.
Carefully flip over the wellington and using the dull side of a butter knife, cut the dough in a criss-cross pattern from side to side. Do not cut through the dough.
Brush the welling with the remaining egg wash to fully coat.
Transfer it to the lined baking sheet and bake for 30 minutes, until the pastry is golden brown. Remove from the oven and let cool for 10 minutes before serving.
Enjoy!