Preheat oven to 375°. In a large bowl, whisk together all ingredients until well mixed. (Fold in mushrooms last.) Spray 9-inch pie plate with cooking spray and fill with the spinach mixture. Bake for 30 minutes, or until browned and set. Let cool for 20 minutes, cut into wedges and serve.
Oven temperature: If using glass pie plate, reduce temperature by 25 degrees.
Cheese: Parmesan - Fresh cheese is a must! Do not use canned. It's also worth it to go ahead and buy the good Italian varieties. Pecorino Romano is my choice.
Menu Suggestions: Especially nice for brunch with fresh fruit salad.
