Preheat oven to 425 degrees F. Fit the crust into a 9- or 9.5-inch pie plate. Wrap in plastic wrap and place in the freezer while you prepare the topping and apples.
For the topping: In a medium bowl, stir together gluten-free flour, walnuts, coconut sugar, and salt. Mix in butter with your fingertips until the mixture resembles wet sand with some bigger chunks of walnuts and butter. Refrigerate the topping while you prepare the filling.
For the filling: In a large bowl, toss apples with coconut sugar, cinnamon, and lemon juice.
Place apples in the chilled pie crust. Spread topping evenly over apples. Bake pie at 425 degrees F for 15 minutes. Reduce heat to 350 degrees F, cover pie loosely with foil (to prevent over-browning), and continue baking until topping is golden and apples are tender, about 50 minutes more. Cool and serve!