Pre-heat oven to 375 F.
Remove the sausages from their casings. Over medium heat, cook sausage meat until well-browned, about 10 minutes. Remove to a bowl with a slotted spoon.
In the sausage fat, cook the mushrooms over medium heat until tender. Remove with a slotted spoon and store with the browned sausage meat.
Put 1 egg yolk aside for the top.
Mix the rest of the eggs, sausage, mushrooms, spinach, mozzarella, ricotta, salt, pepper, and garlic powder.
Roll out a little more than half the pie dough, and place in the bottom of the pie plate.
Spoon the mixture into the pie.
Roll out the rest of the pie dough, and place it over the pie. Cut slits in the top to let the steam escape.
Mix the egg yolk with the water. Brush the top of the pie with the mixture.
Bake in oven at 375 F for 1 hour, or until a knife inserted in the centre comes out clean.
