Use a combination of apples toget both tartness and sweetness into the pie. I always use a couple of Granny Smith, mixed with Gala, Honey Crisp, Golden Delicious, or Macintosh.
Melt the butter in a large pan over medium heat. Add the apple mixture and cook until the apples soften, about 5 minutes. Look for tender, not mushy, results. Reduce the heat to low. In a small bowl, make a slurry of flour and cream, then pour it over the apples. Cook for about 1 minute to thicken slightly. Remove from the heat and set aside (the filling may be refrigerated or frozen at this stage, in an airtight container, after it cools to room temperature).
2 tablespoons unsalted butter, 1/2 cup heavy cream, 3 tablespoons all purpose flour or cornstarch
Make the topping. In a small bowl, whisk the flour, brown sugar, baking soda, and salt. Rub in the butter with your fingers, then add the walnuts, to make pea-sized pieces. Set aside.
1 cup unbleached, all-purpose flour, 1/2 cup light brown sugar, 1/2 teaspoon baking soda, 1/4 teaspoon kosher salt, 6 tablespoons unsalted butter, softened, 1/2 cup walnuts
Arrange an oven rack in the middle position. Preheat the oven to 375°F.
Make the crust. In another medium bowl, whisk the flour and salt. Slowly stir in the oil with a spoon until pea-sized crumbs form. Stir in the cold water 1 tablespoon at a time. On a damp work surface, roll out the dough between two sheets of waxed paper to a circle 2 inches larger than a 9-inch pie plate. Remove the top sheet of paper and invert the crust, still on the other sheet of waxed paper, over the pie plate. Ease the dough down into the plate and slowly peel the paper off. Tuck the overhang of dough under the edge of the crust to form a thick edge. If there are thin areas, cut some of the overhang from elsewhere and tuck it under the thin spot to make it thicker. Crimp the edge decoratively and prick the bottom of the crust in several spots with a fork.
1 3/4 cup unbleached, all purpose flour, 1 teaspoon kosher salt, 1/2 cup neutral oil, 4 tablespoons ice water
Par-bake the crust until pale golden brown, about 10 minutes. Remove from the oven and keep the oven on.
Fill the par-baked crust with the apple filling, then top the filling evenly with the streusel topping. Cover the edges of the crust with a pie guard. Bake until the top is golden brown, about 30 to 40 minutes. Remove from the oven and cool to barely warm or room temperature. Slice and serve.