How to Make It
In a food processor, pulse 1 1/4 cups of the flour and 1/4 teaspoon of the salt. Add 1 stick of the butter and pulse until it is the size of peas. Drizzle on the ice water and pulse until evenly moistened crumbs form; turn out onto a surface and form into a ball. Flatten into a disk, wrap in plastic and refrigerate until firm.
Preheat the oven to 375°. Set a baking sheet on the bottom rack. In a bowl, toss the apples, lemon juice, sugar, 1/4 cup of the flour and the cinnamon.
On a floured surface, roll a disk of the dough to a 13-inch round; fit it into a deep 10-inch glass pie plate and brush the overhang with water. Crimp the overhang. Spoon in the apples.
In a bowl, whisk the remaining 1 cup of flour, the light brown sugar, the baking soda and the remaining 1/4 teaspoon of salt. Add the 6 tablespoons of softened butter and rub the mixture until sandy. Add the walnuts. Press the mixture into clumps and sprinkle over the pie.
Bake the pie in the center of the oven for 1 hour and 10 minutes, until the crust is golden. Cover the edge of the pie if it begins to darken. Let the pie cool for at least 4 hours before serving.