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, pears, 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.
You might think that a pastry chef would never recommend buying pie crusts but hey, its much better to make a pie with a pre-made crust than not to make a pie at all! For the best flavor, always look for the ones made with at least some butter, rather than all shortening.
You can place the crust in a glass pie dish. That way you can check the bottom crust as it bakes and make sure its getting nice and golden which to my mind is the most important step in pie-making. Nothing ruins a delicious pie like this classic combination of autumn fruits and warm spices faster than an undercooked, soggy bottom crust.
You can mix the topping and keep it refrigerated for up to 7 days.