Peel apples and remove core. Cut them into 1/4-inch-thick slices. In a large bowl, pour the lemon juice over the apples and stir until all slices are covered.
In a separate bowl, mix sour cream, sugar, flour, vanilla, salt and egg until blended. Stir the sour cream mix with the apple slices and lemon juice. Place the pie crust into a 9.5″ deep dish pie plate and pour into the crust.
Preheat the oven to 400°
In a medium bowl, mix the flour, walnuts, sugars, cinnamon, and orange peel. Using a pastry cutter or two butter knives, mix in the cold butter until the mix is crumbly.
Bake the filled crust for one hour at 400°. Then, turn the heat back to 350°, remove the pie, and sprinkle the Streusel topping over the apples. Return to the oven and bake another 30 minutes or until the filling bubbles around the edge of the pie plate.
Cool at least an hour to let the filling set up before serving.