Line a 12 x 17-inch sheet pan with pie dough that has been rolled out to a 17 x 20-inch rectangle. Gently pat into the bottom of the jelly roll pan.
Carefully add the apple-cranberry mixture into the pastry-lined pan and gently press the apples and cranberries into an even layer. With a pastry brush carefully brush the edges of the dough with the beaten egg.
With the reserved strips of dough, weave the strips back and forth to create a lattice top: gently thread the strips over and under each other, pulling the strips back and forth as necessary to create the lattice. Add decorative cut leaves and apples, if desired.
Roll out the second half of dough into a rectangle that is 14 x 20. Carefully cover the apple mixture with pastry and press edges together. Crimp with a fork to seal edges.
This slab pie can be prepared 3-4 weeks in advance and frozen. Seriously!!