In a large bowl, whisk together the flour, salt, and sugar. Add cold cubed butter and work it into the flour using a pastry cutter or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Gradually add ice water, one tablespoon at a time, mixing gently until the dough just comes together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
Peel, core, and slice apples into ¼-inch thick slices. In a large bowl, toss apple slices with lemon juice and vanilla extract.
In a separate bowl, combine flour, cinnamon, nutmeg, allspice, and salt. Sprinkle this dry mixture over the apples and toss until evenly coated.
Pour maple syrup over the apples and gently mix to combine.
Combine chopped walnuts, flour, brown sugar, and cinnamon in a medium bowl. Add cold cubed butter and work it into the mixture with your fingertips until crumbly.
Drizzle maple syrup over the mixture and stir until the streusel forms small clumps.
Preheat your oven to 375°F (190°C).
Roll out the chilled pie dough on a floured surface into a 12-inch circle. Transfer to a 9-inch pie plate, gently pressing into the bottom and sides.
Trim excess dough, leaving a 1-inch overhang, then fold and crimp the edges.
Pour the apple mixture into the prepared crust, arranging the slices so they're slightly mounded in the center.
Sprinkle the walnut streusel evenly over the top, making sure to cover all the apples.
Place the pie on a baking sheet to catch any potential overflow. Bake for 20 minutes, then reduce temperature to 350°F (175°C) and continue baking for 30-35 minutes or until the filling is bubbling and the topping is golden brown.
Allow pie to cool for at least 2 hours before slicing to allow the filling to set properly.