Preheat oven to 450 degrees F. Place prepared piecrust in 13 x 9 inch pan. Bake until very lightly brown. Remove from oven. Piecrust can be made the day before.
The above ingredients make a large quiche (13 inch by 9 inch oval baking dish 2 ½ inches deep). A pie plate size quiche uses 5 eggs and ¾ cup of milk. All other ingredients are the same for the smaller quiche.
Bake 45 minutes or until knife inserted in center comes out clean. Let stand 10 minutes before serving.
Reduce oven temperature to 350 degrees F. Squeeze spinach to remove any remaining water. Place spinach, French fried onions, and cheese over crust. Mix together with a fork. Combine sour cream, mustard, salt and pepper in medium bowl. Mix until well blended. Add eggs and blend again. Add the milk and blend again. This blending sequence prevents lumps in the mixture. Pour over vegetable and cheese layers.