Preheat the oven to 425°F (220°C) and line a baking sheet with aluminum foil. Set aside.

Peel, core, and slice the pears ¼-inch (6-mm) thick and add to a large bowl as you go.

Add the grated apple, brown sugar, cornstarch, lemon juice, vanilla extract, and salt to the pears and mix to combine. Set aside.

On a floured surface, roll out one of the pie crusts to a ¼-inch (6-mm) thick circle and line a 9-inch (23-cm) pie tin with it. Trim the edges to hang about ½ inch (12 mm) over the sides.

Fill the crust with the pear mixture and drizzle ¼ cup (2½ oz/71 g) butterscotch sauce over the pears.

Roll out the second crust to a ¼-inch (6-mm) thick circle and slice the circle into 1-inch (2½-cm) wide strips. Lay half the strips in one direction, leaving a ½-inch (12-mm) gap between the strips. Weave the remaining strips through in the opposite direction.

Trim any overhanging lattice strips to be even with the bottom crust and fold the bottom crust over the top edges of the lattice crust. Crimp firmly to seal, brush the crust with the egg wash and sprinkle with the granulated sugar.

Place on the prepared baking sheet and bake for 20 minutes, then lower the temperature to 375°F (190°C) and bake for another 40-50 minutes, until the filling is bubbling. (If the crust starts to over brown before the filling is done, cover loosely with foil for the remainder of the baking time.)

Let cool for at least 4 hours at room temperature to allow the filling to set.

Just before serving, warm the remaining ½ cup (5 oz/142 g) butterscotch sauce and pour it into the gaps in the lattice crust or serve on the side, along with some whipped cream if desired.

Store, loosely covered, at room temperature for up to 2 days.