|If using ready to use pie crust; remove from pan and spread out in a 9 x 13 inch (23 x 33 cm) baking pan. If making homemade pastry; use the same amount as for a 9 inch (23 cm) size. Bake in preheated 450 F (230 C) oven until very lightly brown. Remove from oven. Reduce oven temperature to 350 F (180 C).
Squeeze spinach to remove any remaining water. Layer spinach, french-fried onions and cheese over crust. In a medium size bowl; combine sour cream, eggs, milk, mustard, salt and pepper. Mix until well blended. Pour over vegetables and cheese layers. Bake 45 minutes or until a knife inserted in the centre comes out clean. Let stand about 10 minutes before slicing and serving.