Pour the flour, sugar, cold butter and shortening into the bowl of a food processor. Pulse it about 12 to 15 times, or until the butter is pea-sized and the dough looks crumbly. Then, slowly add in the ice-cold water, pulsing the food processor until the dough pulls from the sides into a large ball. Be careful not to over-mix.

Roll out the pie dough into a large rectangle measuring about 28 inches x 12 inches, and about ⅛ inches thick. Cut off the excess edges using a pizza cutter. Next, cut the dough into 8 rectangles that measure about 7 inches x 5 inches. Roll out the excess scraps into a smaller rectangle that's about 14 inches x 10 inches, and cut out 4 more rectangles. You should now have a total of 12 rectangles. Place them on a baking sheet lined with parchment paper, cover and chill in the refrigerator for 1 hour.

Preheat your oven to 350°F. Remove the chilled rectangles from the refrigerator, along with your Thanksgiving leftovers. Spoon 1 tablespoon of cranberry sauce onto a rectangle. Then, layer on 1 tablespoon of mashed potatoes, 1 tablespoon of stuffing and small bits of turkey. Cover the tart with another dough rectangle, and pinch the edges together to seal. Repeat this step with all of the rectangles until you have 6 pop tarts.

Use a fork to make lines along the edges of the pop tarts (cut off uneven edges to make a clean edge, if desired). Place the pop tarts on a baking sheet lined with parchment paper. Use a toothpick to poke several small holes in each pop tart to let the air out when they bake. Beat together the egg and 1 teaspoon of water in a small bowl, and apply an egg wash on each tart. Bake on 350°F for 25 to 30 minutes, until golden brown.

Once the pop tarts are done baking, drizzle them with leftover gravy that's been reheated. Serve while warm and enjoy!