Make the topping: In a medium bowl, mix the flour, brown sugar, and cinnamon
. Add the butter and, using your fingertips, pinch the ingredients together into a sandy, crumbly mixture. Do not over mix; as soon as the mixture is sandy, cover and refrigerate until ready to use.
Heat the oven to 375 degrees F. If using a refrigerated crust, use it to line a glass pie dish. If using a frozen crust, let thaw at room temperature and gently remove the crust from the aluminum liner. Transfer to a glass pie dish.
In a medium bowl, toss the apples
, sugar, cornstarch, cinnamon, and butter together. Place the filling in the pie crust
. Sprinkle the streusel
topping on top and place the pie on a sheet pan
to catch any juices. Bake for 40 to 50 minutes, until the crust is golden brown and the juices are bubbling. Check the pie after 30 minutes; if the streusel topping is already brown, cover lightly with foil.
Let cool at least 30 minutes before serving. Serve warm or at room temperature.