In a large bowl, mix the sliced apples, sugar, flour, and cinnamon. Pour this mixture into the pie crust.
In another bowl, prepare the streusel topping by combining flour, sugar, and cold butter. Use your fingers to mix until it forms a crumbly texture. Sprinkle this over the apple filling.
Bake in the preheated oven for about 50 minutes, or until the topping is golden brown and the filling is bubbly.