Preheat the oven to 350°F. Lightly grease a pie plate and set aside.
To assemble the pie, take out the larger disc of dough. On a lightly floured surface roll the dough out into a circle. The dough should be around 2 inches wider than the pie plate on all sides. Carefully fold the dough in half then lift it into the pie plate. Unfold the dough and arrange it in the pie plate letting the excess hang over the edges.
Prepare an egg wash by whisking the egg with the water. Set aside. Add the cornstarch to the cooled apple mixture and mix well. Spoon about half of the apple mixture into the pie dough. Drizzle half of the toffee sauce over the apples. Repeat with the remaining apple mixture and toffee sauce.
Roll out the remaining disc of dough into a circle slightly larger than the pie plate. Brush the edge of the pie crust in the pie plate to help the top adhere. At this point you could skip the lattice crust and simply use the top piece whole cutting a couple of slits on top to allow steam to escape.
To make a lattice crust, cut the dough into 10 strips about ¾” wide. Evenly space 5 strips of dough over the filling. Use the wider strips near the middle of the pie and the shorter strips near the edge of the pie. Peel back halfway three offset strips of dough and then place a strip crosswise. Fold the three strips back straight. Repeat by folding the offset two strips back and laying down another strip. Repeat this weaving of the strips until the 10 strips are used up.
Trim the excess pie dough around the pie. Crimp the edges to create an attractive crust and help the crust stay together. Brush the top of the lattice and the crimped edges with the egg wash.
Place the pie in the preheated oven. If desired, place an aluminum foil lined baking sheet on the shelf below the pie to catch any spills. Bake for 50 to 60 minutes or until the filling is bubbling and the pastry is well browned and cooked through. Let the pie cool and set for a bit before slicing.