Prepare your pie crust or use a store-bought one. Preheat the oven to 400F.

In a large mixing bowl, combine your sliced apples, 1/2 cup of sugar, cinnamon, and 2 tablespoons of flour. Mix well to coat the apples evenly.

In another bowl, make the crumb topping by mixing the remaining 1/2 cup of sugar, 3/4 cup of flour, and chopped walnuts. Add the melted butter and mix until it forms a crumbly texture.

Transfer the apple mixture into the prepared pie crust. Spread the crumb topping evenly over the apples.

Bake the pie for 45-50 minutes, until the topping is golden brown. Remove from the oven and let it sit for 10 minutes before serving.