While the pies are baking, prepare the walnut streusel topping. Add the ingredients of the streusel to the bowl of a food processor. Pulse the food processor until you've reached the desired consistency.
(Alternatively, add the softened butter to a large mixing bowl with the flour, sugar, brown sugar, lightly chopped walnuts, cinnamon, and salt. Work the ingredient together using your hands. Knead the ingredients with your fingers until you reach a large crumb consistency.
When the pie, with the filling, is taken out of the oven, move it to a safe work surface for hot temperatures. A wooden cutting board works great. Add the streusel topping to the pie, return to the oven, and bake for 25-30 minutes. The pie should be golden brown and very fragrant.
Remove the pie from the oven and set it on a wire cooling rack and allow it to cool off to almost room temperature before attempting to cut.
Refrigerate any leftover pie, and gently reheat for best results to serve leftovers.