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1
Place the prepared pie pastry shell in the freezer for 15 minutes; preheat oven to 400u00b0.
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2
In a big bowl, whisk the sour cream, sugar, egg, flour, salt, and vanilla until well blended.
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3
Add in the apples; mix well.
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4
Turn the filling into the chilled pie shell; smooth the top with your hands, laying the apple slices down flat.
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5
Place the pie on the center oven rack; bake for 15 minutes; decrease heat to 350u00b0; bake for 30 minutes.
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6
Meanwhile, make the topping: in a big bowl, combine the flour, sugars, cinnamon, salt, and walnuts; add in the butter and stir with a fork, then switch to your hands rubbing the mixture gently between your fingers until it forms uniform crumbs; refrigerate until ready to use.
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7
Remove pie from oven and carefully dump the streusel topping in the center, spreading it evenly over the surface with your hands.
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8
Tamp the topping down lightly.
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9
Return pie to oven, placing it so that the part that faced the back of the oven now faces the front.
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10
Slide a foil-line baking sheet onto the rack below to catch any spills.
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11
Continue baking for about 20 minutes or until golden brown.
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12
Transfer pie to wire rack to cool for at least 2 hours before serving;
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13
You may serve this pie chilled, so after it is thoroughly cooled, cover with loosely tented foil and refrigerate at least 2 hours.
