Prepare Pie Crust: Prepare single pie crust as directed through step 6. Adjust oven rack to lowest position. Preheat oven to 375°.
Prepare Streusel: In medium bowl, combine flour, brown sugar and salt. With fingertips, work in butter until pea-sized crumbs form; stir in walnuts. Freeze streusel 15 minutes.
Meanwhile, prepare Filling: In large bowl, stir together granulated sugar, flour and cinnamon. Peel, core and cut apples into 1⁄4-inch-thick slices. You should have about 9 1⁄2 cups. Add apples and lemon juice to granulated sugar mixture and toss to combine. Spread apple filling into prepared pie crust; sprinkle streusel evenly over filling.
Bake 30 minutes. Reduce heat to 350° and bake 30 minutes longer or until filling bubbles and topping is golden brown. Loosely tent pie with aluminum foil during last 20 minutes of baking to prevent overbrowning. Cool pie on wire rack 1 hour to serve warm, or cool completely.