All ingredients should be cold. Combine all the dry ingredients in a large mixing bowl. Add shortening and butter. Using a pastry blender, cut in the shortening and butter until the mixture resembles course meal.
Drop by drop, add the cold water. Mix in with the fingertips, not with the hands as the palms will warm the dough. Continue mixing water in until the dough begins to hold together without being sticky but not crumbly.
Place dough in plastic wrap. Fold over plastic wrap and press down to form a disk. This will make rolling out easier after chilling. Finish wrapping in plastic and place in the refrigerator for at least one hour.
Lightly spray a deep 9-inch pie pan or 8-1/2 fluted flan pan with vegetable cooking spray. Roll out dough and place in pie plate. Brush egg wash on the inside of pie crust. Return to refrigerator until filling is ready. Makes pastry for 9-inch single-crust pie.
Preheat oven to 450 degrees.
Over medium heat, melt butter. Add onions and saute until translucent. Do not brown. Combine eggs, sour cream, and flour. Add to onion mixture. Season with salt and pepper and pour into chilled pie crust. [Cook briefly until thick.] Top with grated cheeses.
Bake for 20 minutes. Reduce oven temperature to 325 degrees and bake for an additional 20 minutes or until center is set.