Preheat oven to 425 degrees.
Remove one of the pie crusts from it's packaging and place on a lightly floured surface. Using a rolling pin, roll pie crust into a rectangle shape, until it's about 8 x 11 inches (give or take a little).
Gently lift the crust and place onto one half of a 15 x 10 x 1 inch baking pan, folding any extra crust around the edges of the pan under so that they are all even. Repeat with the second pie crust and place on the other half of the baking pan (you will have about a 1" overlap in the middle, but just press it down until it's flush with the rest).
In a large bowl, mix together sugar, flour, cinnamon, nutmeg, allspice, salt, lemon juice and cornstarch.
Fold in the apples and stir until they are all coated.
Spoon the apples into the crust and bake for 30-35 minutes or until the filling is bubbly (just watch that your crust doesn't burn - you could line the edges of the crust with aluminum foil to keep it from burning).
Remove from oven and let cool for 30 minutes.
While the pie cools, mix together powdered sugar and milk in a small bowl. Carefully drizzle glaze over the top of the cooled pie and serve.