Mix flour and salt in a large bowl. Cut in shortening and butter until the particles are the size of small peas.
Combine milk and lemon juice together in a small bowl.
Add oil and milk/lemon mixture to the flour mixture and stir until a soft dough forms. You may need to use your hands to form the mixture into a dough.
Divide the dough into two equal portions. Flatten into a disc and wrap with plastic. Refrigerate 30 minutes.
Preheat the oven to 400 degrees. If you have a pizza stone, place it on the oven rack to preheat.
Roll pie crust on a lightly floured surface. Keep rolling until it is a 13x18" rustic-looking rectangle (about 1/8" thick).
Place the rectangle into a 10x15" jelly roll pan, allowing the excess to hand over the edge of the pan.
Sprinkle crust dust (a combination of 2 Tbsp. sugar and 2 Tbsp. flour) over the pie crust bottom.
Pour cherry filling over the bottom crust.
Roll the second crust out into a rustic-looking 13x18" rectangle.
Place on top of the pie.
Fold the edges of the top and bottom crust back and tuck into the pan.
Use your fingers to pinch and crimp the edges of the pie (or use a fork to crimp).
Cut slits into the top of the pie to allow steam to vent while baking.
Brush the crust lightly with egg wash then sprinkle with sugar.
Bake slab pie for 30 minutes.
Cover/tent loosely with foil and bake for an additional 20-25 minutes.
The top should be GOLDEN and the filling should be bubbling.
Allow the pie to cool for at least 2 hours before slicing.