Unroll the pie crust and arrange it in the bottom of a 9-inch deep dish pie plate.
In a large skillet over medium heat, fry the bacon until crispy. Discard all but 2 tbsp. grease, and drain bacon on paper towels.
Add the onions and mushrooms to the skillet with the bacon grease, and cook for 8-10 minutes, or until soft and golden.
Turn the heat to low, and add the spinach.
Cook 1 minute, or until wilted.
Remove from heat. Make sure that any liquid is drained from the veggies.
In a large bowl, whisk together milk, eggs, egg whites, salt, and pepper.
Stir in veggies, bacon, and cheese.
Pour into pie crust.
Bake at 350 degrees for 35-45 minutes, or until a toothpick inserted near the center comes out clean, and the center doesn't jiggle.
Remove to a wire rack.
Cool for 10-15 minutes before cutting and serving.
Top with Greek yogurt or sour cream.