Pre-heat oven to 375 degrees F. Place your pie crust on a baking sheet and bake for 10 minutes.
Place butter in a nonstick pan over medium-high heat. Add in mushrooms and sauté till tender. Add in baby spinach and cook till wilted. Turn off heat and let cool slightly.
In a medium sized bowl add the eggs and milk then whisk till combined. Season with onion salt, salt, and pepper. Gently stir in the feta cheese.
Pour filling over your partially baked pie crust and bake for 35-40 minutes. Serve warm and enjoy!