Crust: Prepare Papa's Pie Crust recipe according to directions. On a floured surface, roll one disk of dough to fit a deep 9-inch glass pie plate. Take a fork and pierce holes in the bottom of the crust to prevent bubbling. Trim the overhang and flute the edges. Spoon in the apple filling.
Filling: In a large bowl, toss together apples, lemon juice, sugar, flour and cinnamon. Spoon filling into prepared pie plate.
Topping: In a medium bowl, whisk together flour, brown sugar, baking soda and salt. Add softened butter, stir to combine. Add walnuts. Press the mixture into clumps and sprinkle over the pie.
Preheat the oven to 375 degrees. Set a baking sheet on the bottom rack to catch spills. Bake the pie in the center of the oven for 1 hour, until the crust is golden and apples are tender. Cover the pie with foil about halfway through the cooking time, so the top doesn't get too dark.
Allow the pie to cool for at least 4 hours before serving. Refrigerate any leftovers.