Preheat the oven to 375 degrees F. Stir the granola, flour, sugar and salt together in a medium bowl. Work the butter in by hand until you have clumps. Pinch the clumps into teaspoon-sized pieces, and place on a parchment-lined rimmed baking sheet. Bake until the mixture has spread, about 7 minutes; stir with a fork to break up the pieces, then continue to bake until golden brown, 3 to 5 minutes more.
Let cool to room temperature, then sprinkle over the top of the pie. Top with whipped cream. (The streusel can be baked and stored in an airtight container at room temperature for up to 2 days.)
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