Pre-heat oven to 425 degrees F.
Mix brown sugar, corn starch, flour and cinnamon. Set aside.
Peel apples and slice them thinly. In a big bowl toss apples with lemon juice and ginger. Add sugar-flour mixture and mix until apples are coated. Be careful not to sample the slices because you won't be able to stop eating them!
Cover a 9-inch round baking dish with one pastry sheet, fill with apples, add pieces of butter and top with the other pastry sheet. With your fingers or a fork, crimp pastry sheets together to keep the apples inside. Make a few slits in the top to allow steam to escape and sprinkle sugar (if desired). Cover edges of pastry with aluminum foil to prevent over browning and bake for 25 minutes.
After 25 minutes, carefully remove aluminum foil and bake for 20-25 minutes more or until crust is golden and filling is bubbling.
Serve warm and a la mode (with a nice scoop of vanilla ice-cream on top).