Preheat oven to 425 degrees F (220 degrees C). Prick the pie crust all over with fork and bake for 5 minutes.
Place the frozen spinach in microwave safe bowl and cook until thawed. Drain as much of the liquid as you can and then use paper towels to get the spinach as dry as possible.
Beat together the eggs, yogurt or sour cream, flour, and pepper until well mixed. Stir in the spinach. Layer the sliced mushrooms, cheese, and chopped scallions in that order on the bottom of the pie crust. Top with the spinach mixture.
Bake in preheated oven for 15 minutes and then decrease temperature to 350 degrees F (175 degrees C) and bake for 30 additional minutes. Let cool before serving.