Cook sausage over medium-high until lightly browned, about 5-7 minutes, being sure to break up the meat into a fine mince.
2. Step
Remove cooked sausage from pan and wipe away excess fat with a paper towel.
3. Step
Add onion and sauté until softened and translucent, about 2 to 3 minutes. Add mushrooms, salt, and pepper and continue to cook for an additional 2-3 minutes.
4. Step
Add chopped spinach and cook until wilted and no extra liquid has accumulated in the bottom of the pan.
5. Step
NOTE: Do your best to use the excessive moisture of the spinach and mushrooms to deglaze the bottom of the pan, scraping away any browned bits from cooking the sausage earlier.
6. Step
Preheat oven to 350˚F (180˚C).
7. Step
Once cooked, remove the spinach and mushroom mixture from the pan and combine it with the reserved cooked sausage. Stir to mix evenly.
8. Step
Combine eggs and milk. Lightly whisk until just combined.
9. Step
Add sausage and spinach mixture to the bottom of a prepared 9-inch (23cm) pie crust, making sure to spread it evenly.
10. Step
Pour in beaten eggs to fill the rest of the pie crust and top with a generous sprinkle of parmesan cheese.
11. Step
Bake at 350˚F (180˚C) for 25 minutes or until the eggs are set and the top is lightly browned. Allow to cool for 10 minutes before slicing and serving.
