In a large bowl, whisk together the flour, sugar and salt. Add the shortening by cutting it with a pastry blender (or by pulling 2 table knives through all the ingredients in opposite directions), until the shortening is in small pieces the size of small peas. Add the cold water, 1 tablespoon at a time, tossing with a fork until all flour is moistened and the pastry almost cleans the side of the bowl (1 to 2 teaspoons more water can be added if necessary).

The pie crust can be made ahead, hours ahead or even the night before. Keep it chilled in the fridge in the form of a two disks wrapped tight in plastic wrap until ready to roll it out.

In a large bowl mix together the sugar, 1/3 cup flour, cinnamon, nutmeg and salt. Stir in the prepared apples until well coated in the mixture. Let sit while you roll out the pie crust.

Add a large piece of parchment paper to your working surface and flour the area including the rolling pin. Working one pie crust disk at a time roll out each disk of pie crust making one larger in size than the other to fit your sheet pan.  With the help of the piece of parchment paper, flip the pie crust over onto the sheet pan. Gently peel the parchment off of the pie crust and press into the sheet pan to fit. Place the apple filling onto the pie crust in the pan. Place second rolled out pie crust on top of the apple filling. Pinch the two pie crusts together around the edges to prevent leaking and to add a decorative edge. Cut small slits into the top of the crust so steam can escape. Cover the edges of the crust with strips of aluminum foil to prevent over browning. Bake for 40 - 45 minutes.

In a saucepan set to medium high heat stir together the first four ingredients. Stir constantly until it thickens. Add the butter and vanilla. While the mixture is still hot and in a liquid state spread over the slab pie and let the caramel set on the pie for a few hours.