Preheat oven to 350 degrees F (175 degrees C). Place a baking sheet on an oven rack.
Place sliced apples in a large bowl. In a small bowl, combine sugar, cinnamon, and flour. Stir well and pour mixture over apples. Cut half of butter into small pieces and add to apples. Toss apples until thoroughly coated.
Roll out pastry to fit an 8- or 9-inch pie plate. Roll out top crust and set aside. Pour apples into pastry-lined pie pan.
Dot apples with remaining butter. Place second pastry on top. Seal edges and cut steam vents in top crust.
Bake in preheated oven on the baking sheet until apple filling is bubbly and crust is golden brown, 50 minutes to 1 hour.
