For the Pie:
Mix the pumpkin, condensed milk, egg, pumpkin pie spice together and pour into prepared crust. Bake for 15 minutes, then reduce oven to 350°.
For the topping:
Mix together flour, brown sugar, and cinnamon, cut in the butter. Stir in walnuts. Sprinkle over the pie when you reduce the oven. Bake another 40 minutes or until knife comes out clean.
